feat(group): 添加 MCP XML 注入开关

- Group 新增 mcp_xml_inject 字段，控制 Antigravity 平台的 MCP XML 协议注入
- 默认启用，可在分组设置中关闭
- 修复 GetByKeyForAuth 遗漏查询 mcp_xml_inject 字段导致认证缓存值始终为 false 的问题

backend/internal/service/antigravity_gateway_service.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/ctxkey"
 	"github.com/gin-gonic/gin"
 	"github.com/google/uuid"
 )
@@ -552,6 +553,10 @@ func (s *AntigravityGatewayService) getClaudeTransformOptions(ctx context.Contex
 	}
 	opts.EnableIdentityPatch = s.settingService.IsIdentityPatchEnabled(ctx)
 	opts.IdentityPatch = s.settingService.GetIdentityPatchPrompt(ctx)
+
+	if group, ok := ctx.Value(ctxkey.Group).(*Group); ok && group != nil {
+		opts.EnableMCPXML = group.MCPXMLInject
+	}
 	return opts
 }