feat(server): add passthrough proxy for OpenAI-compatible endpoints

* implement proxy handler for various OpenAI API routes
* add error handling for request body and response streaming
* introduce new error response format for API compatibility
* add tests for recover middleware to handle panics gracefully

pkg/metrics/middleware.go

@@ -60,3 +60,9 @@ func (sw *statusWriter) WriteHeader(code int) {
 	sw.status = code
 	sw.ResponseWriter.WriteHeader(code)
 }
+
+func (sw *statusWriter) Flush() {
+	if f, ok := sw.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}