OAuth token refresh hits the wrong endpoint when the auth server lives under a path

Ran into this with a hosted MCP server whose authorization server isn't at the origin root — token endpoint is https://host/oauth2/api/v1/token, not https://host/token.

If a client starts up with a cached-but-expired access token (+ refresh token) and hasn't done discovery yet, async_auth_flow refreshes at the very top — before any 401/metadata discovery. So oauth_metadata is None and _refresh_token uses the fallback urljoin(get_authorization_base_url(server_url), "/token"), i.e. just {scheme}://{netloc}/token. That 404s, _handle_refresh_response clears the tokens, and the flow drops to full interactive auth — which a headless/gateway client can't do. So the server silently disconnects every time the access token expires (mine are 5 min, so… constantly).

Same path-stripping fallback is in _get_token_endpoint, _perform_authorization_code_grant (/authorize) and DCR (/register) — refresh is just the one that bites silently.

Repro (roughly):

• MCP server whose AS metadata puts token_endpoint under a path, not {origin}/token
• log in normally so tokens get cached
• let the access token expire (or clear the expiry), reconnect with a fresh provider
• watch the refresh POST go to https://host/token → 404 → "Token refresh failed" → tokens cleared → it tries to open a browser

Fix looks like: discover metadata before the eager refresh (or stop dropping the issuer path in the fallback). Happy to PR — have a branch that pulls the PRM/ASM discovery out of the 401 branch and runs it before the refresh.

(used some AI help digging into this)

首先跟踪 async_auth_flow 及其到 _refresh_token 的 eager refresh 路径，然后比较 _get_token_endpoint、_perform_authorization_code_grant 和 DCR 中的回退行为。针对元数据端点包含路径的 authorization server，重现缓存的过期 token 场景。完成标准是 refresh 使用发现的 token endpoint，并且不会清除有效 token 或回退到交互式身份验证。