Add check_constraints() helper for constraint checking

[LeMyst]

Summary

• Adds wbi_helpers.check_constraints(), a thin wrapper around the wbcheckconstraints API action provided by the WikibaseQualityConstraints extension.
• Unlike the plnode-based approach originally discussed on the issue (which hardcodes Wikidata-specific constraints), this relies entirely on the constraints configured on the target Wikibase instance itself, so it works the same way against Wikidata or any third-party Wikibase installation that has the extension enabled.
• Accepts one or more entity_id and/or claim_id, plus an optional status filter (e.g. violation, warning), letting callers check constraints for a whole entity or for a specific claim (e.g. right after adding it).
• Adds mock support for wbcheckconstraints in the offline test harness (test/conftest.py) and unit tests in test/test_wbi_helpers.py.

Closes #154

Test plan

• python -m pytest test/ --ignore=test/integration (191 passed)
• New TestCheckConstraints unit tests cover: missing id error, single/multiple entity ids, claim id, status filter