fix: add mutex to mockLogger in errorbus tests to prevent race conditions

The mockLogger's errors slice was being accessed concurrently from
multiple goroutines (the error bus consumer and the test goroutine),
causing race conditions when running tests with the race detector.

Added sync.Mutex to protect the errors slice and proper locking when
accessing it in test assertions.

AGENTS.md

@@ -150,8 +150,10 @@ When working on this project, follow this workflow:
   - Use descriptive branch names (e.g., `feature/epic1-http-server`, `bugfix/auth-token-expiry`, `enhancement/rate-limiting`)
   - Branch names should follow the pattern: `{type}/{epic}-{short-description}` or `{type}/{story-id}-{short-description}`
 - **ALWAYS create a commit** after successfully implementing a feature that:
-  - ✅ Builds successfully (`go build` passes)
-  - ✅ Tests pass (`go test` passes)
+  - ✅ Builds successfully (`make build` passes)
+  - ✅ Tests pass (`make test` passes)
+  - ✅ Lint pass (`make lint` passes)
+  - ✅ fmt-check pass (`make fmt-check` passes)
   - ✅ Meets all acceptance criteria from the story
 - Commit messages should be clear and descriptive, referencing the story/epic when applicable
 - Never commit directly to `main` branch