refactor: 引入 ExportMixin 统一模型导出逻辑，修复配置获取与 thinking 检测

- 新增 ExportMixin，基于排除列表自动收集可导出字段，应用于 Provider/Endpoint/GlobalModel/Model/ProviderAPIKey
- 修复系统配置获取：当 key 存在默认值时不再抛出 404
- 导出配置时增加 API Key / auth_config 解密失败的日志警告
- 修复 Gemini normalizer 的 thinking 检测，优先读取 internal.thinking 标准路径

src/core/api_format/conversion/normalizers/gemini.py

@@ -1846,6 +1846,10 @@ class GeminiNormalizer(FormatNormalizer):
     def _is_antigravity_thinking_enabled(self, internal: InternalRequest) -> bool:
         """Best-effort detection of Claude-style `thinking` flag for Antigravity conversions."""
         try:
+            # 标准路径：跨格式转换时 ClaudeNormalizer 会将 thinking 写入 internal.thinking
+            if internal.thinking and internal.thinking.enabled:
+                return True
+
             extra = internal.extra if isinstance(internal.extra, dict) else {}
             claude_extra = extra.get("claude")
             if not isinstance(claude_extra, dict):