fix(cliproxy): support base_URL-only config credentials and skip stale auth headers

- Preserve API key entries with empty `api_key` when `base_url` is configured, and extend config dedupe/ID logic to include base URL, proxy, prefix, and headers so identities are stable.
- Update config/auth handling so `auth_kind=apikey` is treated as config API-key auth even without an `api_key` field, enabling base_url-only credential records.
- Ensure Gemini/Codex/XAI request path clears `Authorization`/provider auth headers when token is empty to avoid leaking unrelated auth state.