feat: 动态 block 索引分配、OAuth 自动启停联动与 Antigravity 签名注入增强

- Gemini/OpenAI normalizer 改为按实际出现顺序延迟分配 thinking/text block 索引
- OAuth 失效标记时自动停用 Key，清除/刷新成功时自动启用
- Antigravity thought signature 注入支持 tool-specific 签名优先，再回退 session 级签名
- Claude normalizer 对非字符串 thinking block 降级为 UnknownBlock
- Gemini normalizer 反序列化时为 Antigravity 目标补充 signature
- signature_cache _prune 增加超限驱逐
- model/provider 包 __init__ 改为惰性导入避免循环依赖
- build_antigravity_url 复制 query_params 防止修改调用方原始字典
- upstream_fetcher build_all_format_configs 兼容非 EndpointFetchConfig 对象
- Antigravity HTTP 状态判定扩展与 Gemini endpoint check 支持 v1internal
- 新增 thought signature 注入与 signature cache 驱逐测试

Closes #165

Co-authored-by: AAEE86 <ppk0227@hotmail.com>

frontend/src/features/providers/components/ProviderDetailDrawer.vue

@@ -1518,12 +1518,13 @@ async function handleClearOAuthInvalid(key: EndpointAPIKey) {
   clearingOAuthInvalidKeyId.value = key.id
   try {
     await clearOAuthInvalid(key.id)
-    showSuccess('已清除 OAuth 异常标记')
+    showSuccess('已清除 OAuth 异常标记，Key 已自动启用')
     // 更新本地数据
     const keyInList = providerKeys.value.find(k => k.id === key.id)
     if (keyInList) {
       keyInList.oauth_invalid_at = null
       keyInList.oauth_invalid_reason = null
+      keyInList.is_active = true
     }
     await loadEndpoints()
   } catch (err: any) {