fix: complete OAuth-UA salvage follow-up (stale comment + test keychain isolation)

Two review findings on the #57922 salvage:

1. Stale inline comment at the login-exchange site still claimed the token
   endpoint uses the claude-code/ UA prefix and 404s claude-cli/ — now
   contradicts the axios/ fix. Repointed it at _OAUTH_TOKEN_USER_AGENT.

2. The inherited Path.home test isolation on the three TestRefreshOauthToken
   tests only stubbed the ~/.claude *file* source, not the macOS Keychain.
   _refresh_oauth_token re-reads read_claude_code_credentials() (keychain
   first) in its adopt-already-refreshed branch, so on any macOS dev/CI runner
   with real Claude Code creds the branch short-circuits and the 3 tests fail.
   Stub read_claude_code_credentials -> None so the tests are hermetic.

(The remaining TestResolveAnthropicToken/TestResolveWithRefresh/TestRunOauthSetupToken
failures on macOS are the same pre-existing keychain-leak class on origin/main,
unrelated to this OAuth-UA fix, and pass in CI — left out of scope.)

agent/anthropic_adapter.py

@@ -1488,8 +1488,9 @@ def run_hermes_oauth_login_pure() -> Optional[Dict[str, Any]]:
         # Anthropic migrated the OAuth token endpoint to platform.claude.com;
         # console.anthropic.com now 404s. Try the new host first, then fall
         # back to console for older deployments (mirrors the refresh path).
-        # Use the claude-code/ UA prefix: Anthropic blocks claude-cli/ on the
-        # OAuth token endpoint (returns 404 for all versions).
+        # UA is _OAUTH_TOKEN_USER_AGENT (a non-claude-code UA) — see the
+        # constant's definition for why the token endpoint must not send
+        # claude-code/ (429 UA-prefix block).
         result = None
         last_error = None
         for endpoint in _OAUTH_TOKEN_URLS:

tests/agent/test_anthropic_adapter.py

@@ -509,11 +509,20 @@ class TestResolveAnthropicToken:
 class TestRefreshOauthToken:
     def test_returns_none_without_refresh_token(self, tmp_path, monkeypatch):
         monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        # Neutralize live Claude Code sources (macOS Keychain + ~/.claude file)
+        # so the adopt-already-refreshed branch can't short-circuit with a real
+        # credential on a dev/CI machine that happens to have Claude Code creds.
+        monkeypatch.setattr(
+            "agent.anthropic_adapter.read_claude_code_credentials", lambda: None
+        )
         creds = {"accessToken": "expired", "refreshToken": "", "expiresAt": 0}
         assert _refresh_oauth_token(creds) is None
 
     def test_successful_refresh(self, tmp_path, monkeypatch):
         monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        monkeypatch.setattr(
+            "agent.anthropic_adapter.read_claude_code_credentials", lambda: None
+        )
 
         creds = {
             "accessToken": "old-token",
@@ -547,6 +556,9 @@ class TestRefreshOauthToken:
 
     def test_failed_refresh_returns_none(self, tmp_path, monkeypatch):
         monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
+        monkeypatch.setattr(
+            "agent.anthropic_adapter.read_claude_code_credentials", lambda: None
+        )
         creds = {
             "accessToken": "old",
             "refreshToken": "refresh-123",