fix(web_server): hold _oauth_sessions_lock during PKCE session state writes

_submit_anthropic_pkce() retrieved sess under _oauth_sessions_lock but
wrote back to sess["status"] and sess["error_message"] outside the lock.
A concurrent session GC or cancel could race with these writes, producing
inconsistent session state.

Wrap all 4 sess write sites in _oauth_sessions_lock:
- network exception path (Token exchange failed)
- missing access_token path
- credential save failure path
- success path (approved)

hermes_cli/web_server.py

@@ -1533,6 +1533,7 @@ def _submit_anthropic_pkce(session_id: str, code_input: str) -> Dict[str, Any]:
         with urllib.request.urlopen(req, timeout=20) as resp:
             result = json.loads(resp.read().decode())
     except Exception as e:
+        with _oauth_sessions_lock:
             sess["status"] = "error"
             sess["error_message"] = f"Token exchange failed: {e}"
         return {"ok": False, "status": "error", "message": sess["error_message"]}
@@ -1541,6 +1542,7 @@ def _submit_anthropic_pkce(session_id: str, code_input: str) -> Dict[str, Any]:
     refresh_token = result.get("refresh_token", "")
     expires_in = int(result.get("expires_in") or 3600)
     if not access_token:
+        with _oauth_sessions_lock:
             sess["status"] = "error"
             sess["error_message"] = "No access token returned"
         return {"ok": False, "status": "error", "message": sess["error_message"]}
@@ -1549,9 +1551,11 @@ def _submit_anthropic_pkce(session_id: str, code_input: str) -> Dict[str, Any]:
     try:
         _save_anthropic_oauth_creds(access_token, refresh_token, expires_at_ms)
     except Exception as e:
+        with _oauth_sessions_lock:
             sess["status"] = "error"
             sess["error_message"] = f"Save failed: {e}"
         return {"ok": False, "status": "error", "message": sess["error_message"]}
+    with _oauth_sessions_lock:
         sess["status"] = "approved"
     _log.info("oauth/pkce: anthropic login completed (session=%s)", session_id)
     return {"ok": True, "status": "approved"}