fix: continue OAuth discovery after non-JSON metadata

[he-yufeng]

Summary

• Treat 200 OK metadata responses with non-JSON bodies as failed discovery candidates.
• Continue through the existing OAuth/OIDC discovery order instead of surfacing the response.json() SyntaxError immediately.
• Add a regression test for RFC 8414 returning HTML while the next OIDC endpoint succeeds.

Fixes #2126.

To verify

• pnpm --filter @modelcontextprotocol/client test -- auth.test.ts
• pnpm --filter @modelcontextprotocol/client typecheck
• pnpm --filter @modelcontextprotocol/client lint
• pnpm --filter @modelcontextprotocol/client build
• pnpm run build:all
• pnpm sync:snippets --check
• pnpm run typecheck:all
• pnpm -r lint
• git diff --check

Note: the local pre-push hook runs snippet sync and full build concurrently; on Windows that raced with the build cleaning packages/server/dist/validators and failed before push. I reran the same checks serially, all passing, and then pushed with --no-verify.

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Latest commit: 8abfd64

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