fix(分组): 强化上下文分组可信校验

- 引入 Hydrated 标记限制复用来源

- 无效上下文分组允许被新值覆盖自愈

- 更新相关单测覆盖

backend/internal/service/gateway_multiplatform_test.go

@@ -1072,6 +1072,7 @@ func TestGatewayService_GroupResolution_ReusesContextGroup(t *testing.T) {
 		ID:       groupID,
 		Platform: PlatformAnthropic,
 		Status:   StatusActive,
+		Hydrated: true,
 	}
 	ctx = context.WithValue(ctx, ctxkey.Group, group)
 
@@ -1106,8 +1107,9 @@ func TestGatewayService_GroupResolution_IgnoresInvalidContextGroup(t *testing.T)
 	ctx := context.Background()
 	groupID := int64(42)
 	ctxGroup := &Group{
-		ID:     groupID,
-		Status: StatusActive,
+		ID:       groupID,
+		Platform: PlatformAnthropic,
+		Status:   StatusActive,
 	}
 	ctx = context.WithValue(ctx, ctxkey.Group, ctxGroup)
 
@@ -1153,6 +1155,7 @@ func TestGatewayService_GroupResolution_FallbackUsesLiteOnce(t *testing.T) {
 		Status:          StatusActive,
 		ClaudeCodeOnly:  true,
 		FallbackGroupID: &fallbackID,
+		Hydrated:        true,
 	}
 	fallbackGroup := &Group{
 		ID:       fallbackID,

backend/internal/service/gateway_service.go

@@ -643,7 +643,7 @@ func (s *GatewayService) withGroupContext(ctx context.Context, group *Group) con
 	if !IsGroupContextValid(group) {
 		return ctx
 	}
-	if existing, ok := ctx.Value(ctxkey.Group).(*Group); ok && existing != nil && existing.ID == group.ID {
+	if existing, ok := ctx.Value(ctxkey.Group).(*Group); ok && existing != nil && existing.ID == group.ID && IsGroupContextValid(existing) {
 		return ctx
 	}
 	return context.WithValue(ctx, ctxkey.Group, group)

backend/internal/service/gemini_multiplatform_test.go

@@ -261,6 +261,7 @@ func TestGeminiMessagesCompatService_GroupResolution_ReusesContextGroup(t *testi
 		ID:       groupID,
 		Platform: PlatformGemini,
 		Status:   StatusActive,
+		Hydrated: true,
 	}
 	ctx = context.WithValue(ctx, ctxkey.Group, group)
 

backend/internal/service/group.go

@@ -10,6 +10,7 @@ type Group struct {
 	RateMultiplier float64
 	IsExclusive    bool
 	Status         string
+	Hydrated       bool // indicates the group was loaded from a trusted repository source
 
 	SubscriptionType    string
 	DailyLimitUSD       *float64
@@ -81,6 +82,9 @@ func IsGroupContextValid(group *Group) bool {
 	if group.ID <= 0 {
 		return false
 	}
+	if !group.Hydrated {
+		return false
+	}
 	if group.Platform == "" || group.Status == "" {
 		return false
 	}