MCP OAuth: AS discovery uses bare /.well-known/oauth-authorization-server instead of RFC 8414 path-based URL

## Summary

When Claude Code discovers the Authorization Server metadata for an MCP connector, it constructs the discovery URL by doing:

```js
new URL('/.well-known/oauth-authorization-server', issuerUrl)
```

For an issuer with a path (e.g. `https://gateway.example.com/github`), this strips the path and fetches:
```
https://gateway.example.com/.well-known/oauth-authorization-server
```

Per RFC 8414 Section 3, the correct URL for a path-based issuer is:
```
https://gateway.example.com/.well-known/oauth-authorization-server/github
```

## Impact

Any MCP gateway that serves per-server AS metadata at the RFC 8414 canonical path will have working metadata at `/.well-known/oauth-authorization-server/github` (with `registration_endpoint`) but the SDK fetches the bare URL which returns generic metadata without `registration_endpoint`. Claude Code then errors:

```
Incompatible auth server: does not support dynamic client registration
```

## Reproduction

1. Protected resource at `/.well-known/oauth-protected-resource/github/mcp` returns `authorization_servers: ["https://gateway/github"]`
2. Claude Code fetches `https://gateway/.well-known/oauth-authorization-server` (no `registration_endpoint`)
3. Error fires

The correct URL `https://gateway/.well-known/oauth-authorization-server/github` is never fetched even though it returns valid metadata with `registration_endpoint`.

## Expected behavior

For issuer `https://gateway/github` (has path `/github`), the AS discovery URL should be:
`https://gateway/.well-known/oauth-authorization-server/github`

This is RFC 8414 Section 3 compliant: `{issuer_host}/.well-known/oauth-authorization-server{issuer_path}`

## Verified in

Claude Code 2.1.275 — binary only contains the bare path string `/.well-known/oauth-authorization-server`, no path-appended variant.

Start by locating the Authorization Server discovery URL construction described in the Claude Code binary and compare it with RFC 8414 Section 3. Reproduce the path-based issuer case using the URLs in the report; done means the canonical path-based URL is fetched and metadata including registration_endpoint is recognized.

Written by the indexing model from the issue text.