Tests for the responseWriter struct in middleware.go:
- APIError.Error() interface implementation (3 cases)
- WriteHeader once-only semantics (4 cases)
- Write with implicit WriteHeader (3 cases)
- StatusCode nil safety (4 cases)
- Hijack support detection (2 cases)
- Flush support detection (3 cases)
- Full flow integration and edge cases (4 cases)
First test file for middleware.go. 22 test cases total covering
HTTP response wrapper behavior used by ErrorHandler middleware.
