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>

src/services/orchestration/error_classifier.py

@@ -747,9 +747,10 @@ class ErrorClassifier:
 
                     key.oauth_invalid_at = datetime.now(timezone.utc)
                     key.oauth_invalid_reason = f"{OAUTH_ACCOUNT_BLOCK_PREFIX}Google 要求验证账号"
+                    key.is_active = False
                     self.db.commit()
                     logger.warning(
-                        "  [{}] {} 因 403 VALIDATION_REQUIRED 已标记为账号异常",
+                        "  [{}] {} 因 403 VALIDATION_REQUIRED 已标记为账号异常并自动停用",
                         request_id,
                         self._format_key_display(key),
                     )