rmcp-client: fix auth crash (#11692)
Don't load auth tokens if bearer token is present. This fixes a crash I
was getting on Linux:
```
2026-02-12T23:26:24.999408Z DEBUG session_init: codex_core::codex: Configuring session: model=gpt-5.3-codex-spark; provider=ModelProviderInfo { name: "OpenAI", base_url: None, env_key: None, env_key_instructions: No
ne, experimental_bearer_token: None, wire_api: Responses, query_params: None, http_headers: Some({"version": "0.0.0"}), env_http_headers: Some({"OpenAI-Project": "OPENAI_PROJECT", "OpenAI-Organization": "OPENAI_ORGA
NIZATION"}), request_max_retries: None, stream_max_retries: None, stream_idle_timeout_ms: None, requires_openai_auth: true, supports_websockets: true }
2026-02-12T23:26:24.999799Z TRACE session_init: codex_keyring_store: keyring.load start, service=Codex MCP Credentials, account=codex_apps|20398391ad12d90b
thread 'tokio-runtime-worker' (96190) has overflowed its stack
fatal runtime error: stack overflow, aborting
Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.35s
```
395729910c