feat(antigravity): 添加混合调度可选功能

- 后端：账户模型添加 IsMixedSchedulingEnabled() 方法，读取 extra.mixed_scheduling
- 后端：gateway_service 和 gemini_messages_compat_service 支持混合调度逻辑
- 后端：分组创建支持指定 platform 参数
- 前端：账户创建/编辑弹窗添加混合调度开关（仅 antigravity 账户显示）
- 前端：混合调度开关添加问号图标和 tooltip 说明
- 前端：GroupSelector 支持根据 mixedScheduling 属性过滤分组
- 前端：分组创建支持选择 platform
- 测试：e2e 测试添加 ENDPOINT_PREFIX 环境变量支持混合/隔离模式测试
- 测试：删除过时的 Claude signature 测试用例

frontend/src/views/admin/GroupsView.vue

@@ -78,11 +78,21 @@
                   ? 'bg-orange-100 text-orange-700 dark:bg-orange-900/30 dark:text-orange-400'
                   : value === 'openai'
                     ? 'bg-emerald-100 text-emerald-700 dark:bg-emerald-900/30 dark:text-emerald-400'
-                    : 'bg-blue-100 text-blue-700 dark:bg-blue-900/30 dark:text-blue-400'
+                    : value === 'antigravity'
+                      ? 'bg-purple-100 text-purple-700 dark:bg-purple-900/30 dark:text-purple-400'
+                      : 'bg-blue-100 text-blue-700 dark:bg-blue-900/30 dark:text-blue-400'
               ]"
             >
               <PlatformIcon :platform="value" size="xs" />
-              {{ value === 'anthropic' ? 'Anthropic' : value === 'openai' ? 'OpenAI' : 'Gemini' }}
+              {{
+                value === 'anthropic'
+                  ? 'Anthropic'
+                  : value === 'openai'
+                    ? 'OpenAI'
+                    : value === 'antigravity'
+                      ? 'Antigravity'
+                      : 'Gemini'
+              }}
             </span>
           </template>
 
@@ -604,14 +614,16 @@ const exclusiveOptions = computed(() => [
 const platformOptions = computed(() => [
   { value: 'anthropic', label: 'Anthropic' },
   { value: 'openai', label: 'OpenAI' },
-  { value: 'gemini', label: 'Gemini' }
+  { value: 'gemini', label: 'Gemini' },
+  { value: 'antigravity', label: 'Antigravity' }
 ])
 
 const platformFilterOptions = computed(() => [
   { value: '', label: t('admin.groups.allPlatforms') },
   { value: 'anthropic', label: 'Anthropic' },
   { value: 'openai', label: 'OpenAI' },
-  { value: 'gemini', label: 'Gemini' }
+  { value: 'gemini', label: 'Gemini' },
+  { value: 'antigravity', label: 'Antigravity' }
 ])
 
 const editStatusOptions = computed(() => [