Add compatibility test for projection operators (3 total)

[eerxuan]

core operator projection - - elemMatch
core operator projection - - positional
core operator projection - - slice

[eerxuan]

Status update — broken into sub-issues

Already covered (comprehensive tests exist)

• positional — documentdb_tests/compatibility/tests/core/operator/projection/positional

Sub-issues created for remaining features

• Add compatibility test for $elemMatch #359 — elemMatch (smoke-only — needs comprehensive tests)
• Add compatibility test for $slice (projection) #360 — slice (smoke-only — needs comprehensive tests)

This issue is now a tracking parent — see the tracking label. Filter -label:tracking to see only the actual work items.

[eerxuan]

All features listed in this story now have comprehensive compatibility tests (beyond smoke tests). Closing as complete.