## Why
Enterprise sign-in needs to keep browser completion separate from credential storage so callers can recheck the active account and configuration before saving a grant. Logout must also prevent an earlier sign-in from restoring credentials, including from another process sharing `CODEX_HOME`.
## What changed
- Add enterprise login APIs in `rmcp-client` that return an authorization URL and stage validated credentials for an explicit `commit_if` call. Store grants only in the keyring after rechecking caller authority under the credential lock.
- Require a registered client ID, published metadata matching the configured issuer, HTTP loopback callbacks, a refresh token, and a valid OIDC identity assertion. Request `openid` and `offline_access` with `prompt=consent`, and omit MCP resource indicators from authorization and code exchange.
- Persist a login generation under the credential lock so logout invalidates pending and staged sign-ins across processes, even when no grant is stored.
- Keep credentials and account identifiers out of enterprise error chains and logs, and avoid logging callback payloads when the receiver has closed.
## Testing
Add coverage for discovery validation, loopback callbacks, PKCE, staged keyring storage, cancellation, stale attempts, cross-process logout, and error/log privacy. Preserve ordinary MCP OAuth login without a refresh token, and adjust the terminal polling test deadline to include the minimum empty-poll wait.
GitOrigin-RevId: 2a27b9a26505a2f6fdecce8877f6e2c21e148f72
b090e901f8