Map reasoning_effort to Anthropic output_config.effort (#115)

This PR adds support for mapping the unified `reasoning_effort` field to
Anthropic's `output_config.effort` API parameter, which is the
recommended way to control thinking depth for Claude Opus 4.6 and Sonnet
4.6 models. Previously, the Anthropic provider silently dropped
`reasoning_effort` from requests, while the OpenAI provider already
correctly mapped it to `reasoning: { effort }`. This inconsistency meant
that workflow nodes setting `reasoning_effort: "high"` (the default) had
no effect when routing through Anthropic.

The change adds an `output_config: Option<serde_json::Value>` field to
the `ApiRequest` struct with `skip_serializing_if = "Option::is_none"`
to ensure it's omitted when not set, then populates it in
`build_api_request` by transforming `request.reasoning_effort` into
`{"effort": <value>}` — mirroring the pattern used in the OpenAI
provider. All existing `ApiRequest` constructions in tests are updated
to include `output_config: None`.

The PR also takes the opportunity to refactor the test module by
extracting a `make_base_request()` helper, which reduces boilerplate in
`build_api_request_omits_whitespace_only_system_prompt` and
`make_request_with_format` and makes the two new tests
(`build_api_request_maps_reasoning_effort_to_output_config` and
`build_api_request_omits_output_config_when_no_reasoning_effort`) easy
to read. All existing provider tests continue to pass and no new Clippy
warnings are introduced.

### Fabro Details

<details>
<summary>Ran 9 stages in 10m 28s for $1.48</summary>

| Stage | Duration | Cost | Retries |
|---|---|---|---|
| start | 0s | – | 0 |
| toolchain | 0s | – | 0 |
| preflight_compile | 1m 8s | – | 0 |
| preflight_lint | 12s | – | 0 |
| implement | 2m 13s | $0.49 | 0 |
| simplify_opus | 4m 45s | $0.99 | 0 |
| simplify_gpt | 0s | – | 0 |
| verify | 1m 40s | – | 0 |
| fmt | 1s | – | 0 |
| **Total** | **10m 28s** | **$1.48** | **0** |

</details>

<details>
<summary>Ran <code>ImplementAndSimplify.fabro</code> (12 nodes and 15
edges)</summary>

```dot
digraph ImplementAndSimplify {
    graph [
        goal="Implement and simplify",
        model_stylesheet="
            * { backend: api; model: claude-opus-4-6;}
        "
    ]
    rankdir=LR

    start [shape=Mdiamond, label="Start"]
    exit  [shape=Msquare, label="Exit"]

    toolchain         [label="Toolchain", shape=parallelogram, script="command -v cargo >/dev/null || { curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y && sudo ln -sf $HOME/.cargo/bin/* /usr/local/bin/; }; cargo --version 2>&1", max_retries=0]
    preflight_compile [label="Preflight Compile", shape=parallelogram, script="cargo check -q --workspace 2>&1", max_retries=0]
    preflight_lint    [label="Preflight Lint", shape=parallelogram, script="cargo clippy -q --workspace -- -D warnings 2>&1", max_retries=0]
    fix_lints         [label="Fix Lints", prompt="The preflight lint step failed. Read the build output from context and fix all clippy lint warnings.", max_visits=3]
    implement         [label="Implement", prompt="Read the plan file referenced in the goal and implement every step. Make all the code changes described in the plan. Use red/green TDD."]
    simplify_opus     [label="Simplify (Opus)", prompt="@prompts/simplify.md"]
    simplify_gpt      [label="Simplify (GPT-54)", prompt="@prompts/simplify.md", model="gpt-54"]
    verify            [label="Verify", shape=parallelogram, script="cargo clippy -q --workspace -- -D warnings 2>&1 && cargo nextest run --cargo-quiet --workspace --status-level fail 2>&1", goal_gate=true, retry_target="fixup"]
    fixup             [label="Fixup", prompt="The verify step failed. Read the build output from context and fix all clippy lint warnings and test failures.", max_visits=3]
    fmt               [label="Format", shape=parallelogram, script="cargo fmt --all 2>&1", max_retries=0]

    start -> toolchain
    toolchain -> preflight_compile [condition="outcome=success"]
    toolchain -> exit
    preflight_compile -> preflight_lint [condition="outcome=success"]
    preflight_compile -> exit
    preflight_lint -> implement [condition="outcome=success"]
    preflight_lint -> fix_lints
    fix_lints -> preflight_lint
    implement -> simplify_opus -> simplify_gpt -> verify
    verify -> fmt   [condition="outcome=success"]
    verify -> fixup
    fixup -> verify
    fmt -> exit
}

```

</details>

⚒️ Generated with [Fabro](https://fabro.sh)

---------

Co-authored-by: Fabro <noreply@fabro.sh>

lib/crates/fabro-llm/src/providers/anthropic.rs

@@ -107,6 +107,8 @@ struct ApiRequest {
     #[serde(skip_serializing_if = "Option::is_none")]
     thinking: Option<serde_json::Value>,
     #[serde(skip_serializing_if = "Option::is_none")]
+    output_config: Option<serde_json::Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     metadata: Option<std::collections::HashMap<String, String>>,
     #[serde(skip_serializing_if = "std::ops::Not::not")]
     stream: bool,
@@ -1072,6 +1074,11 @@ fn build_api_request(
 
     let thinking = extract_thinking_config(request.provider_options.as_ref());
 
+    let output_config = request
+        .reasoning_effort
+        .as_ref()
+        .map(|effort| serde_json::json!({"effort": effort}));
+
     let api_request = ApiRequest {
         model: request.model.clone(),
         messages: api_messages,
@@ -1088,6 +1095,7 @@ fn build_api_request(
         tools: api_tools,
         tool_choice: tool_choice_json,
         thinking,
+        output_config,
         metadata: request.metadata.clone(),
         stream,
     };
@@ -1577,6 +1585,7 @@ mod tests {
             tools: None,
             tool_choice: None,
             thinking: None,
+            output_config: None,
             metadata: None,
             stream: false,
         };
@@ -1592,8 +1601,21 @@ mod tests {
     fn build_api_request_omits_whitespace_only_system_prompt() {
         let adapter = Adapter::new("test-key");
         let request = Request {
-            model: "claude-sonnet-4-20250514".to_string(),
             messages: vec![Message::system("   \n\t"), Message::user("Hello")],
+            ..make_base_request()
+        };
+
+        let (api_request, _req_builder) = build_api_request(&adapter, &request, false);
+        assert!(
+            api_request.system.is_none(),
+            "whitespace-only system prompts should be omitted"
+        );
+    }
+
+    fn make_base_request() -> Request {
+        Request {
+            model: "claude-sonnet-4-20250514".to_string(),
+            messages: vec![Message::user("Hello")],
             provider: Some("anthropic".to_string()),
             tools: None,
             tool_choice: None,
@@ -1605,30 +1627,15 @@ mod tests {
             reasoning_effort: None,
             metadata: None,
             provider_options: None,
-        };
-
-        let (api_request, _req_builder) = build_api_request(&adapter, &request, false);
-        assert!(
-            api_request.system.is_none(),
-            "whitespace-only system prompts should be omitted"
-        );
+        }
     }
 
     fn make_request_with_format(format: crate::types::ResponseFormat) -> Request {
         Request {
-            model: "claude-sonnet-4-20250514".to_string(),
-            messages: vec![Message::user("Hello")],
             provider: None,
-            tools: None,
-            tool_choice: None,
             response_format: Some(format),
-            temperature: None,
-            top_p: None,
             max_tokens: None,
-            stop_sequences: None,
-            reasoning_effort: None,
-            metadata: None,
-            provider_options: None,
+            ..make_base_request()
         }
     }
 
@@ -1973,6 +1980,7 @@ mod tests {
             tools: None,
             tool_choice: None,
             thinking: None,
+            output_config: None,
             metadata: None,
             stream: false,
         };
@@ -2004,6 +2012,7 @@ mod tests {
             tools: None,
             tool_choice: None,
             thinking: None,
+            output_config: None,
             metadata: None,
             stream: false,
         };
@@ -2040,4 +2049,28 @@ mod tests {
             "[Audio content not supported by this provider]"
         );
     }
+
+    #[test]
+    fn build_api_request_maps_reasoning_effort_to_output_config() {
+        let adapter = Adapter::new("test-key");
+        let request = Request {
+            reasoning_effort: Some("medium".to_string()),
+            ..make_base_request()
+        };
+
+        let (api_request, _req_builder) = build_api_request(&adapter, &request, false);
+        assert_eq!(
+            api_request.output_config,
+            Some(serde_json::json!({"effort": "medium"}))
+        );
+    }
+
+    #[test]
+    fn build_api_request_omits_output_config_when_no_reasoning_effort() {
+        let adapter = Adapter::new("test-key");
+        let request = make_base_request();
+
+        let (api_request, _req_builder) = build_api_request(&adapter, &request, false);
+        assert!(api_request.output_config.is_none());
+    }
 }