Add system variable tests for $$ROOT

[vic-tsang]

This PR contains:

• system variable tests for $$ROOT

Ref:

• Issue Add test coverage for aggregation system variables #117

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, documentation, enhancement
Project fields suggested: Component test-coverage · Priority P3 · Effort L · Status Needs Review
Confidence: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, docs); effort from diff stats (1059+1 LOC, 13 files); LLM: Adds new test cases for $$ROOT system variable, which is a test coverage improvement with no functional code changes.

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