fix(csharp): index C# 14 extension declaration members

[lorenzozanee]

What does this PR do?

C# 14 extension declarations are parsed as constructor-shaped nodes by the pinned grammar, so methods inside them are omitted from the index. Recover nested extension members during C# class extraction and add focused regression coverage.

Fixes #2071

Checklist

• Every commit is signed off (git commit -s) - required, CI rejects unsigned
  commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (scripts/test.sh --suites "grammar_regression grammar_labels extraction cs_lsp")
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.

[DeusData]

Thank you for taking on C# 14 extension members and including both grammar-label and extraction regression coverage. This is queued as a high-priority parsing fix for #2071. We need more time to review the extraction behavior before giving a decision.

The review queue is currently full, so detailed feedback may take a little time. We are working through it carefully and appreciate the work you have put into supporting the project. Thank you for your patience.