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/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)