Replace mockito with HttpClient trait in fabro-github and detect import self-loops

Introduce an HttpClient trait abstraction over reqwest::Client so tests
use a lightweight MockHttpClient instead of spawning a TCP server via
mockito. This removes the mockito dev-dependency entirely and makes
tests faster and more deterministic.

Also add self-loop detection in ImportTransform to poison placeholders
that have edges pointing back to themselves.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1673,7 +1673,6 @@ dependencies = [
  "base64",
  "chrono",
  "jsonwebtoken",
- "mockito",
  "reqwest",
  "serde",
  "serde_json",

lib/crates/fabro-github/Cargo.toml

@@ -21,6 +21,5 @@ tracing.workspace = true
 tokio = { workspace = true }
 
 [dev-dependencies]
-mockito = "1"
 tokio = { workspace = true, features = ["test-util", "macros"] }
 base64.workspace = true

lib/crates/fabro-workflows/src/transforms/import.rs

@@ -88,6 +88,19 @@ impl ImportTransform {
             return;
         }
 
+        if graph
+            .edges
+            .iter()
+            .any(|edge| edge.from == placeholder_id && edge.to == placeholder_id)
+        {
+            Self::poison_placeholder(
+                graph,
+                placeholder_id,
+                &format!("import placeholder '{placeholder_id}' cannot have a self-loop"),
+            );
+            return;
+        }
+
         let placeholder = match Self::placeholder_config(graph, placeholder_id) {
             Ok(placeholder) => placeholder,
             Err(message) => {