feat(codex): 引入 upstream_headers hook 机制，为 Codex 注入 session/conversation/account headers

- 新增 upstream_headers.py：可注册 provider+endpoint 维度的 extra headers 构建 hook
- Codex openai:cli 注入 session_id + conversation_id（由 prompt_cache_key sha256 派生）
- Codex openai:compact 注入 chatgpt-account-id（来自 auth_config）+ session_id，不注入 conversation_id
- 修复 prompt_cache：compact 格式现统一为 codex 策略，不再跳过注入
- chat_handler_base / cli_request_mixin 均在 extra_headers 阶段调用 build_upstream_extra_headers

tests/services/test_upstream_headers.py

@@ -0,0 +1,95 @@
+from __future__ import annotations
+
+import hashlib
+
+from src.services.provider.adapters.codex.context import (
+    CodexRequestContext,
+    set_codex_request_context,
+)
+from src.services.provider.upstream_headers import build_upstream_extra_headers
+
+
+def test_build_upstream_extra_headers_for_codex_openai_cli() -> None:
+    request_body = {"model": "gpt-5", "input": [], "prompt_cache_key": "pcache-123"}
+
+    headers = build_upstream_extra_headers(
+        provider_type="codex",
+        endpoint_sig="openai:cli",
+        request_body=request_body,
+        original_headers={},
+        decrypted_auth_config={"account_id": "acc-1"},
+    )
+
+    short_id = hashlib.sha256(b"pcache-123").hexdigest()[:16]
+    assert headers == {
+        "chatgpt-account-id": "acc-1",
+        "session_id": short_id,
+        "conversation_id": short_id,
+    }
+
+
+def test_build_upstream_extra_headers_respects_existing_session_headers() -> None:
+    headers = build_upstream_extra_headers(
+        provider_type="codex",
+        endpoint_sig="openai:cli",
+        request_body={"model": "gpt-5", "input": [], "prompt_cache_key": "pcache-123"},
+        original_headers={
+            "Session_ID": "client-session",
+            "Conversation_ID": "client-conversation",
+        },
+        decrypted_auth_config={"account_id": "acc-1"},
+    )
+
+    assert headers == {"chatgpt-account-id": "acc-1"}
+
+
+def test_build_upstream_extra_headers_for_codex_openai_compact() -> None:
+    request_body = {"model": "gpt-5", "input": [], "prompt_cache_key": "pcache-123"}
+
+    headers = build_upstream_extra_headers(
+        provider_type="codex",
+        endpoint_sig="openai:compact",
+        request_body=request_body,
+        original_headers={},
+        decrypted_auth_config={"account_id": "acc-1"},
+    )
+
+    short_id = hashlib.sha256(b"pcache-123").hexdigest()[:16]
+    assert headers == {
+        "chatgpt-account-id": "acc-1",
+        "session_id": short_id,
+    }
+
+
+def test_build_upstream_extra_headers_for_legacy_codex_compact_context() -> None:
+    request_body = {"model": "gpt-5", "input": [], "prompt_cache_key": "pcache-123"}
+
+    try:
+        set_codex_request_context(CodexRequestContext(is_compact=True))
+        headers = build_upstream_extra_headers(
+            provider_type="codex",
+            endpoint_sig="openai:cli",
+            request_body=request_body,
+            original_headers={},
+            decrypted_auth_config={"account_id": "acc-1"},
+        )
+    finally:
+        set_codex_request_context(None)
+
+    short_id = hashlib.sha256(b"pcache-123").hexdigest()[:16]
+    assert headers == {
+        "chatgpt-account-id": "acc-1",
+        "session_id": short_id,
+    }
+
+
+def test_build_upstream_extra_headers_returns_empty_without_match() -> None:
+    headers = build_upstream_extra_headers(
+        provider_type="codex",
+        endpoint_sig="openai:chat",
+        request_body={"model": "gpt-5", "messages": []},
+        original_headers={},
+        decrypted_auth_config={"account_id": "acc-1"},
+    )
+
+    assert headers == {}