Add $floor arithmetic expression tests

[danielfrankcom]

This change adds tests for the $floor arithmetic expression. It was originally authored by @vic-tsang.

Closes #276

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort L · Status Needs Review
Confidence: 0.30 (deterministic)

Reasoning

component from path globs (test-coverage); effort from diff stats (646+0 LOC, 6 files); LLM failed: Invalid response body while trying to fetch https://api.anthropic.com/v1/messages: Premature close

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[PatersonProjects]

Missing __init__.py

May want to consider adding dedicated bare argument test: {"$floor": "$value"}, though the case is covered by smoke.

May want to add a nested field path test case.

[danielfrankcom]

Missing __init__.py

Not strictly needed here, won't add it for consistency with the other expression dirs