refactor(codex): 移除 envelope/request_patching 层，用 context var 统一 compact 状态判断

- 删除 CodexOAuthEnvelope 和 request_patching 模块，Codex 不再需要 envelope 层
- 移除 _aether_compact 请求体内部标记，改用 is_codex_compact_request() 集中查询
- 简化 OpenAI CLI adapter，移除 Codex 专用的 get_cli_extra_headers/build_test_request_body 逻辑
- 移除 Codex behavior variant 注册（same_format/cross_format）
- normalizer patch_same_format_request 对 codex 变为 no-op
- 更新相关测试适配新的架构

tests/api/handlers/base/test_cli_request_body_isolation.py

@@ -51,7 +51,6 @@ class _DummyCliStreamHandler(CliStreamMixin):
         return out
 
     def prepare_provider_request_body(self, request_body: dict[str, Any]) -> dict[str, Any]:
-        request_body.pop("_aether_compact", None)
         request_body["input"][0]["content"][0]["text"] = "prepared"
         return request_body
 
@@ -116,7 +115,6 @@ async def test_execute_stream_request_does_not_mutate_original_request_body(
 
     original_request_body = {
         "model": "gpt-test",
-        "_aether_compact": True,
         "input": [
             {
                 "role": "user",
@@ -141,6 +139,5 @@ async def test_execute_stream_request_does_not_mutate_original_request_body(
 
     assert original_request_body == snapshot
     assert handler._request_builder.request_body is not None
-    assert "_aether_compact" not in handler._request_builder.request_body
     assert handler._request_builder.request_body["input"][0]["content"][0]["text"] == "prepared"
     assert handler._request_builder.request_body["input"][0]["content"][-1]["text"] == "finalized"