Fix custom criteria membership checks

[Kkkakania]

Summary

Make custom criteria that match variables in the current object appear in obj.cf.keys(). Because CFAccessor.__contains__ delegates to keys(), membership checks such as "salt" in ds.cf now agree with successful ds.cf["salt"] lookups.

Custom vocabulary entries with no match are intentionally omitted, preserving the documented meaning of keys() as the set of valid keys for the current object.

Closes #375.
Closes #531.

Tests

• Added a regression test covering matched and unmatched custom criteria through both keys() and in.
• PYTHONPATH=. ../cf-xarray-127/.venv/bin/python -m pytest cf_xarray/tests -q (284 passed, 1 skipped)
• uvx pre-commit run --files cf_xarray/accessor.py cf_xarray/tests/test_accessor.py

---

AI disclosure: this change was AI-assisted. I reproduced the failure before the implementation, reviewed the final diff, and ran the full local test suite and pre-commit hooks.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.03%. Comparing base (a9cebee) to head (973b53e).
⚠️ Report is 126 commits behind head on main.

Files with missing lines	Patch %	Lines
cf_xarray/accessor.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #659      +/-   ##
==========================================
+ Coverage   85.78%   87.03%   +1.25%     
==========================================
  Files          13       16       +3     
  Lines        2364     3325     +961     
  Branches      183      309     +126     
==========================================
+ Hits         2028     2894     +866     
- Misses        303      384      +81     
- Partials       33       47      +14     

Flag	Coverage Δ
mypy	46.18% <20.00%> (+7.65%)	⬆️
unittests	92.62% <100.00%> (-1.37%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.