mcpAuthRouter ignores path of baseUrl

**Example code**
```
authRouter.use(mcpAuthRouter({
    baseUrl: new URL('https://example.com/auth'),
    // ...
  }));
```

**Expected behavior**
OAuth endpoints should start with `https://example.com/auth/` (i.e. `https://example.com/auth/authorize`).
However, createOAuthMetadata uses absolute paths for endpoints, which causes the `/auth/` prefix to be omitted.
