Add test coverage for aggregation system variables

[eerxuan]

Summary

Add comprehensive test coverage for aggregation system variables. This includes feature tree entries, smoke tests, and detailed compatibility tests.

System Variables to Cover

Variable	Context	Priority
$$ROOT	Full current document	High
$$CURRENT	Current document (equivalent to field paths)	High
$$REMOVE	Remove field from output	High
$$NOW	Current datetime	High
$$CLUSTER_TIME	Current cluster time (replica set)	Medium
$$DESCEND	$redact stage	Medium
$$PRUNE	$redact stage	Medium
$$KEEP	$redact stage	Medium
$$USER_ROLES	Current user roles	Low

Tasks

• Add feature tree entries under core,operator,expressions,variable,system-variables
• Add smoke tests for each supported variable
• Add shared expression engine tests under expressions/variable/system-variables/:
  • $$ROOT — in $project, $addFields, nested paths ($$ROOT.a.b), inside $cond
  • $$CURRENT — equivalence with field paths, in various expression contexts
  • $$REMOVE — in $project, $addFields, conditional removal ($cond + $$REMOVE), nested fields
  • $$NOW — type verification, consistency within a pipeline, usage in date expressions
  • $$DESCEND/$$PRUNE/$$KEEP — with $redact stage
  • $$CLUSTER_TIME — type verification, availability

Test Location

documentdb_tests/compatibility/tests/core/operator/expressions/variable/system-variables/

Notes

• $let variable scoping is already covered in PR Add $let and $literal tests #102
• Per TEST_COVERAGE.md, system variables are shared expression engine behavior — test once with a representative operator, not per-operator