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/antigravity/test_signature_cache.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+from src.services.antigravity.constants import DUMMY_THOUGHT_SIGNATURE
+from src.services.antigravity.signature_cache import ThinkingSignatureCache
+
+
+def test_get_or_dummy_returns_dummy_for_gemini_models() -> None:
+    cache = ThinkingSignatureCache(maxsize=10)
+    assert cache.get_or_dummy("gemini-3-pro", "thinking...") == DUMMY_THOUGHT_SIGNATURE
+
+
+def test_get_or_dummy_returns_none_for_non_gemini_models() -> None:
+    cache = ThinkingSignatureCache(maxsize=10)
+    assert cache.get_or_dummy("claude-sonnet", "thinking...") is None
+
+
+def test_cached_signature_preferred() -> None:
+    cache = ThinkingSignatureCache(maxsize=10)
+    cache.cache("gemini-3-pro", "t", "sig-1")
+    assert cache.get_or_dummy("gemini-3-pro", "t") == "sig-1"
+
+
+def test_eviction_fifo() -> None:
+    cache = ThinkingSignatureCache(maxsize=4)
+    cache.cache("gemini-3-pro", "t1", "s1")
+    cache.cache("gemini-3-pro", "t2", "s2")
+    cache.cache("gemini-3-pro", "t3", "s3")
+    cache.cache("gemini-3-pro", "t4", "s4")
+
+    # Trigger eviction (evict 1 key when maxsize=4)
+    cache.cache("gemini-3-pro", "t5", "s5")
+
+    # Oldest key should be gone
+    assert cache.get_or_dummy("gemini-3-pro", "t1") == DUMMY_THOUGHT_SIGNATURE
+