✨ feat(mq-lang): add assert_type, assert_field, assert_matches to test module

[harehare]

Summary

Adds a node/domain assertion API to the test module: assert_type checks a markdown node kind (or runtime type for scalars), assert_field checks a required dict key, and assert_matches checks a value's string form against a regex. All three return the value on success or an error dict with a source range and message on failure, so callers can point at the exact node that failed a constraint.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor
• 📝 Documentation
• ⚡ Performance
• ✅ Test
• 📦 Build / dependencies
• 👷 CI

Checklist

• I ran cargo fmt and cargo clippy and addressed any warnings
• I ran just test-all and all tests pass
• I added or updated tests covering this change
• I updated relevant documentation (/docs, crate README.md) if needed
• I added a changelog entry if this is a user-facing change

Additional Context

[codspeed-hq]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing feat/assert-node-api (7b77fae) with main (7652a84)}