fix: address PR review - Anthropic error format in panic recovery and nil guard

- Add recoverAnthropicMessagesPanic for Messages handler to return
  Anthropic-formatted errors instead of OpenAI Responses format on panic
- Add nil check for rateLimitService.HandleUpstreamError in
  ForwardAsAnthropic to match defensive pattern used elsewhere
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/openai_gateway_handler.go

@@ -428,7 +428,7 @@ func (h *OpenAIGatewayHandler) logOpenAIRemoteCompactOutcome(c *gin.Context, sta
 // POST /v1/messages (when group platform is OpenAI)
 func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 	streamStarted := false
-	defer h.recoverResponsesPanic(c, &streamStarted)
+	defer h.recoverAnthropicMessagesPanic(c, &streamStarted)
 
 	requestStart := time.Now()
 
@@ -1157,6 +1157,26 @@ func (h *OpenAIGatewayHandler) recoverResponsesPanic(c *gin.Context, streamStart
 	)
 }
 
+// recoverAnthropicMessagesPanic recovers from panics in the Anthropic Messages
+// handler and returns an Anthropic-formatted error response.
+func (h *OpenAIGatewayHandler) recoverAnthropicMessagesPanic(c *gin.Context, streamStarted *bool) {
+	recovered := recover()
+	if recovered == nil {
+		return
+	}
+
+	started := streamStarted != nil && *streamStarted
+	requestLogger(c, "handler.openai_gateway.messages").Error(
+		"openai.messages_panic_recovered",
+		zap.Bool("stream_started", started),
+		zap.Any("panic", recovered),
+		zap.ByteString("stack", debug.Stack()),
+	)
+	if !started {
+		h.anthropicErrorResponse(c, http.StatusInternalServerError, "api_error", "Internal server error")
+	}
+}
+
 func (h *OpenAIGatewayHandler) ensureResponsesDependencies(c *gin.Context, reqLog *zap.Logger) bool {
 	missing := h.missingResponsesDependencies()
 	if len(missing) == 0 {

backend/internal/service/openai_gateway_messages.go

@@ -137,7 +137,9 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 				Message:            upstreamMsg,
 				Detail:             upstreamDetail,
 			})
-			s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
+			if s.rateLimitService != nil {
+				s.rateLimitService.HandleUpstreamError(ctx, account, resp.StatusCode, resp.Header, respBody)
+			}
 			return nil, &UpstreamFailoverError{StatusCode: resp.StatusCode, ResponseBody: respBody}
 		}
 		// Non-failover error: return Anthropic-formatted error to client