fix: sync Antigravity ForwardResult.Usage with client response simulation

Apply Claude Max cache billing to usage before returning ForwardResult
in Antigravity Forward, ensuring RecordUsage gets the same simulated
usage that clients see. Restore apply+fallback in RecordUsage for
consistency across GatewayService and Antigravity paths.

backend/internal/service/antigravity_gateway_service.go

@@ -1619,6 +1619,9 @@ func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context,
 		firstTokenMs = streamRes.firstTokenMs
 	}
 
+	// Claude Max cache billing: 同步 ForwardResult.Usage 与客户端响应体一致
+	applyClaudeMaxCacheBillingPolicyToUsage(usage, parsedRequestFromGinContext(c), claudeMaxGroupFromGinContext(c), originalModel, account.ID)
+
 	return &ForwardResult{
 		RequestID:        requestID,
 		Usage:            *usage,