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/core/test_provider_oauth_utils_antigravity.py

@@ -0,0 +1,38 @@
+from __future__ import annotations
+
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from src.core.provider_oauth_utils import enrich_auth_config
+
+
+@pytest.mark.asyncio
+async def test_enrich_auth_config_antigravity_adds_project_id_and_email() -> None:
+    auth_config: dict[str, object] = {}
+    token_response: dict[str, object] = {}
+
+    with (
+        patch("src.core.provider_oauth_utils.fetch_google_email", AsyncMock(return_value="u@example.com")),
+        patch(
+            "src.services.antigravity.client.load_code_assist",
+            AsyncMock(
+                return_value={
+                    "cloudaicompanionProject": "project-1",
+                    "currentTier": {"tierType": "PAID"},
+                }
+            ),
+        ),
+    ):
+        out = await enrich_auth_config(
+            provider_type="antigravity",
+            auth_config=auth_config,  # in-place
+            token_response=token_response,
+            access_token="tok",
+            proxy_config=None,
+        )
+
+    assert out["email"] == "u@example.com"
+    assert out["project_id"] == "project-1"
+    assert out["tier"] == "PAID"
+