fix(gateway): complete MarkResponseCommitted coverage for all platforms

Add MarkResponseCommitted to 7 error-writing helper functions and
4 early-return branches in OpenAI handlers, plus 3 inline c.Data paths.

Helper functions (1 line each, covers 60+ call sites):
- writeGatewayCCError, writeResponsesError (Anthropic compat)
- writeClaudeError, writeGoogleError (Gemini + Antigravity)
- writeChatCompletionsError (Gemini Chat Completions compat)

OpenAI handleErrorResponse/handleCompatErrorResponse:
- passthrough rule and ShouldHandleErrorCode branches return before
  the existing MarkResponseCommitted, now each has its own Mark.

Inline c.Data: Gemini ForwardNative (2) + Antigravity ForwardGemini (1).

backend/internal/service/antigravity_gateway_service.go

@@ -2447,6 +2447,7 @@ func (s *AntigravityGatewayService) ForwardGemini(ctx context.Context, c *gin.Co
 			Detail:             upstreamDetail,
 		})
 		logger.LegacyPrintf("service.antigravity_gateway", "[antigravity-Forward] upstream error status=%d body=%s", resp.StatusCode, truncateForLog(unwrappedForOps, 500))
+		MarkResponseCommitted(c)
 		c.Data(resp.StatusCode, contentType, unwrappedForOps)
 		return nil, fmt.Errorf("antigravity upstream error: %d", resp.StatusCode)
 	}
@@ -3644,6 +3645,7 @@ func mergeTextPartsToResponse(response map[string]any, textParts []string) map[s
 }
 
 func (s *AntigravityGatewayService) writeClaudeError(c *gin.Context, status int, errType, message string) error {
+	MarkResponseCommitted(c)
 	c.JSON(status, gin.H{
 		"type":  "error",
 		"error": gin.H{"type": errType, "message": message},
@@ -3735,6 +3737,7 @@ func (s *AntigravityGatewayService) writeMappedClaudeError(c *gin.Context, accou
 }
 
 func (s *AntigravityGatewayService) writeGoogleError(c *gin.Context, status int, message string) error {
+	MarkResponseCommitted(c)
 	statusStr := "UNKNOWN"
 	switch status {
 	case 400:

backend/internal/service/gateway_forward_as_chat_completions.go

@@ -498,6 +498,7 @@ func (s *GatewayService) handleCCStreamingFromAnthropic(
 // writeGatewayCCError writes an error in OpenAI Chat Completions format for
 // the Anthropic-upstream CC forwarding path.
 func writeGatewayCCError(c *gin.Context, statusCode int, errType, message string) {
+	MarkResponseCommitted(c)
 	c.JSON(statusCode, gin.H{
 		"error": gin.H{
 			"type":    errType,

backend/internal/service/gateway_forward_as_responses.go

@@ -520,6 +520,7 @@ func appendRawJSON(existing json.RawMessage, fragment string) json.RawMessage {
 
 // writeResponsesError writes an error response in OpenAI Responses API format.
 func writeResponsesError(c *gin.Context, statusCode int, code, message string) {
+	MarkResponseCommitted(c)
 	c.JSON(statusCode, gin.H{
 		"error": gin.H{
 			"code":    code,

backend/internal/service/gemini_messages_compat_service.go

@@ -1449,6 +1449,7 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 				if contentType == "" {
 					contentType = "application/json"
 				}
+				MarkResponseCommitted(c)
 				c.Data(http.StatusInternalServerError, contentType, respBody)
 				return nil, fmt.Errorf("gemini upstream error: %d (skipped by error policy)", resp.StatusCode)
 			case ErrorPolicyMatched, ErrorPolicyTempUnscheduled:
@@ -1561,6 +1562,7 @@ func (s *GeminiMessagesCompatService) ForwardNative(ctx context.Context, c *gin.
 		if contentType == "" {
 			contentType = "application/json"
 		}
+		MarkResponseCommitted(c)
 		c.Data(resp.StatusCode, contentType, respBody)
 		if upstreamMsg == "" {
 			return nil, fmt.Errorf("gemini upstream error: %d", resp.StatusCode)
@@ -2244,6 +2246,7 @@ func randomHex(nBytes int) string {
 }
 
 func (s *GeminiMessagesCompatService) writeClaudeError(c *gin.Context, status int, errType, message string) error {
+	MarkResponseCommitted(c)
 	c.JSON(status, gin.H{
 		"type":  "error",
 		"error": gin.H{"type": errType, "message": message},
@@ -2252,6 +2255,7 @@ func (s *GeminiMessagesCompatService) writeClaudeError(c *gin.Context, status in
 }
 
 func (s *GeminiMessagesCompatService) writeGoogleError(c *gin.Context, status int, message string) error {
+	MarkResponseCommitted(c)
 	c.JSON(status, gin.H{
 		"error": gin.H{
 			"code":    status,

backend/internal/service/openai_gateway_chat_completions.go

@@ -853,6 +853,7 @@ func (s *OpenAIGatewayService) handleChatStreamingResponse(
 
 // writeChatCompletionsError writes an error response in OpenAI Chat Completions format.
 func writeChatCompletionsError(c *gin.Context, statusCode int, errType, message string) {
+	MarkResponseCommitted(c)
 	c.JSON(statusCode, gin.H{
 		"error": gin.H{
 			"type":    errType,

backend/internal/service/openai_gateway_service.go

@@ -4290,6 +4290,7 @@ func (s *OpenAIGatewayService) handleErrorResponse(
 		"upstream_error",
 		"Upstream request failed",
 	); matched {
+		MarkResponseCommitted(c)
 		c.JSON(status, gin.H{
 			"error": gin.H{
 				"type":    errType,
@@ -4317,6 +4318,7 @@ func (s *OpenAIGatewayService) handleErrorResponse(
 			Message:            upstreamMsg,
 			Detail:             upstreamDetail,
 		})
+		MarkResponseCommitted(c)
 		c.JSON(http.StatusInternalServerError, gin.H{
 			"error": gin.H{
 				"type":    "upstream_error",
@@ -4441,6 +4443,7 @@ func (s *OpenAIGatewayService) handleCompatErrorResponse(
 		c, account.Platform, resp.StatusCode, body,
 		http.StatusBadGateway, "api_error", "Upstream request failed",
 	); matched {
+		MarkResponseCommitted(c)
 		writeError(c, status, errType, errMsg)
 		if upstreamMsg == "" {
 			upstreamMsg = errMsg
@@ -4464,6 +4467,7 @@ func (s *OpenAIGatewayService) handleCompatErrorResponse(
 			Message:            upstreamMsg,
 			Detail:             upstreamDetail,
 		})
+		MarkResponseCommitted(c)
 		writeError(c, http.StatusInternalServerError, "api_error", "Upstream gateway error")
 		if upstreamMsg == "" {
 			return nil, fmt.Errorf("upstream error: %d (not in custom error codes)", resp.StatusCode)