fix(search): honor the label filter in BM25 keyword search (#2148)

[htarnacki]

What does this PR do?

Fixes #2148.

search_graph honours label on the name-pattern and semantic branches but not on the BM25 keyword branch, so query + label returned nodes of every label. The label is now bound into both BM25 statements (FTS lookup and the fallback), matching the other search paths. One commit, MCP-level test in test_mcp.c (Route vs same-named Function).

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test) — all suites green except 10 test_cli.c install/uninstall cases that fail identically on a clean main in my environment (activation transaction I/O failed: target-entry-snapshot, environment-specific).
• Lint passes (make -f Makefile.cbm lint-ci) — clang-format-20 and check-dco.sh clean; clang-tidy / cppcheck are not installed locally, relying on CI for those two.
• 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.