feat: Antigravity 和 Codex 服务支持

- 新增 Antigravity 服务：签名缓存、URL 可用性检测、信封处理
- 新增 Codex 服务：信封处理、元数据收集器
- 重构 provider transport 支持新的服务架构
- 新增 stream_bridge 和 upstream_stream_bridge 处理流式响应
- 优化 OAuth 工具函数
- 添加相关测试用例

tests/services/test_provider_transport_codex.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from types import SimpleNamespace
+
+from src.services.provider.transport import build_provider_url
+
+
+@dataclass
+class _DummyEndpoint:
+    base_url: str
+    api_format: str
+    custom_path: str | None = None
+    provider: object | None = None
+
+
+def test_codex_openai_cli_uses_responses_path_without_v1_prefix() -> None:
+    endpoint = _DummyEndpoint(
+        base_url="https://chatgpt.com/backend-api/codex",
+        api_format="openai:cli",
+        provider=SimpleNamespace(provider_type="codex"),
+    )
+
+    url = build_provider_url(
+        endpoint,  # type: ignore[arg-type] - test stub
+        path_params={"model": "ignored"},
+        is_stream=True,
+    )
+
+    assert url == "https://chatgpt.com/backend-api/codex/responses"
+
+
+def test_codex_openai_cli_does_not_duplicate_responses_suffix() -> None:
+    endpoint = _DummyEndpoint(
+        base_url="https://chatgpt.com/backend-api/codex/responses",
+        api_format="openai:cli",
+        provider=SimpleNamespace(provider_type="codex"),
+    )
+
+    url = build_provider_url(
+        endpoint,  # type: ignore[arg-type] - test stub
+        path_params={"model": "ignored"},
+        is_stream=False,
+    )
+
+    assert url == "https://chatgpt.com/backend-api/codex/responses"
+