Add $rank and $denseRank window operator compatibility tests

[thanh-ttt]

Add $rank and $denseRank window operator compatibility tests

Adds comprehensive test coverage for $rank and $denseRank under $setWindowFields, following the same patterns as $documentNumber. Three test files were added for each operator (6 files total, 76 tests):

File	Dimension
test_window_*_argument_validation.py	Asserting valid syntax (1. Of the form { $denseRank: { } } or { $rank: { } }, 2. Must have only 1 valid sortBy value, 3. No window can be specified)
test_window_*_order_dependence.py	Ascending vs descending sort produces different rankings, ranking restarts per partition, omitted partitionBy treats collection as one partition
test_window_*_tie_handling.py	No ties, all ties, partial ties at beginning/middle/end, multiple tie groups

Key behavioral differences tested:

• $rank: Tied documents share the same rank, next rank skips positions (1, 1, 3)
  • Documentation: https://www.mongodb.com/docs/manual/reference/operator/aggregation/rank/
• $denseRank: Tied documents share the same rank, next rank is dense / no gap (1, 1, 2)
  • Documentation: https://www.mongodb.com/docs/manual/reference/operator/aggregation/denseRank/

[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: 0.95 (mixed)

Reasoning

component from path globs (test-coverage); effort from diff stats (1183+0 LOC, 6 files); LLM: Adds 6 new compatibility test files (76 tests) covering $rank and $denseRank window operators under $setWindowFields, expanding test coverage for important aggregation functionality.

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[eerxuan]

The empty-collection error tests (test_rank_window_key_errors_on_empty_collection etc.) assert the same error codes as their populated counterparts, differing only to prove the error is parse-time — a code-path distinction rather than observable behavior.