Add $setWindowFields sortBy validation corner case tests

[Cassie-2025]

Summary

• Add tests for sortBy-related corner cases in $setWindowFields stage validation:
  • Range window with sortBy: null (treated as omitted, rejected with error 5339902)
  • Range window with sortBy: {} (empty object, no sort fields, rejected with error 5339902)
  • Time-range window with fractional bounds (e.g., [-1.5, 1.5] — must be integers, rejected with
    FAILED_TO_PARSE_ERROR)
  • Time-range window with single fractional lower bound (confirms either bound being fractional triggers error)
  • Bounded documents window with sortBy: null (rejected with error 5339901)
• Update TEST_COVERAGE.md to document these requirements for sortBy, window.range, and window.unit

Test plan

• All 111 tests under setWindowFields/ pass against MongoDB 8.2.4 (port 27018)
• Pre-commit hooks pass (black, isort, flake8, mypy)
• New tests follow existing patterns and use framework assertions

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, documentation, enhancement
Project fields suggested: Component test-coverage · Priority P3 · Effort M · Status Needs Review
Confidence: 0.90 (mixed)

Reasoning

component from path globs (test-coverage, docs); effort from diff stats (149+3 LOC, 4 files); LLM: Adds corner-case validation tests for $setWindowFields sortBy and updates TEST_COVERAGE.md; purely additive test coverage with no functional or schema changes.

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