test(server): isolate storage in streaming_session_turn_updates_runtime_context test (#293)

## Summary

- Fixes the flaky nightly release-pipeline failure observed on tag
`v0.235.0-nightly.1` ([run
25975240763](https://github.com/fabro-sh/fabro/actions/runs/25975240763/job/76354137058)).
-
`streaming_session_turn_updates_runtime_context_without_copying_prior_history_to_turn`
was using the default test server settings, which means every parallel
test shares the default session storage directory
(`$HOME/.fabro/storage`).
- `session_store::write_json` writes via `fs::write`, which truncates
the file before writing. A concurrent reader from another test's
`AppState` session lookup can observe the empty file mid-write and fail
to deserialize. The deserialization error bubbled up as a `turn.failed`
SSE event carrying `Serialization error: EOF while parsing a value at
line 1 column 0`.
- Apply the same isolation pattern used in `e9387bf62` for
`interrupt_active_session_turn_cancels_runtime_and_persists_interrupted`:
give this test its own storage root under `std::env::temp_dir()`.

## Test plan

- [x] `cargo nextest run -p fabro-server -- streaming_session_turn`
passes locally
- [x] `cargo nextest run -p fabro-server` (full suite, 552 tests) passes
locally
- [x] `cargo +nightly-2026-04-14 clippy -p fabro-server --all-targets --
-D warnings` clean
- [ ] CI green

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/crates/fabro-server/src/server/tests.rs

@@ -2529,15 +2529,31 @@ async fn streaming_session_turn_updates_runtime_context_without_copying_prior_hi
         })
         .await;
     let openai_base_url = llm.url("/v1");
-    let state = test_app_state_with_env_lookup(
-        default_test_server_settings(),
-        RunLayer::default(),
-        5,
-        move |name| match name {
-            "OPENAI_BASE_URL" => Some(openai_base_url.clone()),
-            _ => None,
-        },
-    );
+    // Use an isolated storage root so parallel tests do not race on the
+    // shared default session storage directory. `session_store` writes
+    // `session.json` via `fs::write`, which truncates before writing; a
+    // concurrent reader can observe the empty file and fail to deserialize.
+    let storage_dir = std::env::temp_dir().join(format!("fabro-server-test-{}", Ulid::new()));
+    std::fs::create_dir_all(&storage_dir).expect("test storage dir should be creatable");
+    let server_settings = server_settings_from_toml(&format!(
+        r#"
+_version = 1
+
+[server.storage]
+root = "{}"
+
+[server.auth]
+methods = ["dev-token"]
+"#,
+        storage_dir.display()
+    ));
+    let state =
+        test_app_state_with_env_lookup(server_settings, RunLayer::default(), 5, move |name| {
+            match name {
+                "OPENAI_BASE_URL" => Some(openai_base_url.clone()),
+                _ => None,
+            }
+        });
     state
         .vault
         .write()