fix: continue OAuth discovery after non-JSON metadata

[he-yufeng]

Summary

• continue authorization server discovery when a 200 OK metadata response is not JSON
• keep the existing 4xx/502 fallback behavior and non-502 5xx failure behavior
• add a regression test for falling through from the OAuth metadata URL to the OIDC metadata URL

Fixes #2126

To verify

• pnpm --filter @modelcontextprotocol/client test -- test/client/auth.test.ts
• pnpm --filter @modelcontextprotocol/client typecheck
• pnpm --filter @modelcontextprotocol/client lint

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