fix(auth): forward User-Agent header to OAuth discovery and flow requests (#3531)

[Theater-ahyeon]

Description

When OAuthClientProvider (via RedirectAwareAuth) issues OAuth metadata discovery and registration requests, they are sent as bare httpx2.Request instances that do not go through client.build_request(). Consequently, these requests omit the User-Agent header.

When interacting with MCP servers protected by WAFs (such as Cloudflare bot protection), user-agent-less requests are rejected with 403 Forbidden, causing the OAuth flow to fail.

This PR forwards the User-Agent header from the authenticated request to intermediate auth flow requests if not already set.

Closes #3531

Verification

• Added unit test test_auth_flow_forwards_user_agent_header in tests/client/test_auth.py.
• Ran pytest tests/client/test_auth.py (all 161 tests passed).
• Checked formatting and lint with ruff check and ruff format.

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3531.

If a maintainer assigns you to #3531, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.