Add $facet tests

[PatersonProjects]

This pr adds tests for the $facet stage operator

Ref: Issue #47, #485

[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: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (1755+0 LOC, 12 files); LLM: Adds new compatibility tests for the $facet aggregation stage operator, expanding test coverage for an existing feature.

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[fanyangv]

The $lookup test only covers the equi-join form (localField/foreignField). Missing: $lookup with pipeline form inside $facet — this tests nested sub-pipelines ($facet → $lookup → pipeline → [stages]). Consider adding:

1. Success tests with pipeline-form $lookup containing multiple stages (e.g., $match, $project, $sort, $group, $unwind, $addFields, etc.)
2. Error tests with all $facet-forbidden stages ($out, $merge, $collStats, $indexStats, $planCacheStats, $geoNear, $facet) inside the $lookup pipeline to verify they're still rejected when nested inside $facet

[fanyangv]

Many success/behavior tests (across the whole PR) use a single sub-pipeline. $facet is designed for multiple sub-pipelines on the same input — single-sub-pipeline is a special/minimal case. Consider adding a second sub-pipeline (e.g., "first": [{"$limit": 1}]) to the success cases in test_facet_subpipeline_stages.py, test_stages_position_facet.py, and test_facet_collation.py to make them more realistic.

[fanyangv]

Per the container-feature rule (one test per sub-feature), these stages are valid inside $facet but not tested: $redact, $sample, $unionWith, $setWindowFields. Each should have one smoke case confirming it works inside a $facet sub-pipeline.

[fanyangv]

Missing field name edge cases that should be verified (as either accepted or rejected): spaces ("my field"), hyphens ("my-field"), underscores ("my_field"), numeric-only ("123"), at-sign ("@field"), null byte ("a\x00b"). These should be tested to document the actual behavior — whether they succeed or error.

[fanyangv]

Missing the allowDiskUse: false case. Both true and false should be tested to confirm $facet operates normally regardless of the flag value.

[fanyangv]

Please add two tests:

1. $sample before $facet — two sub-pipelines that each return all documents and verify they see exactly the same data
2. $addFields with $rand before $facet — two sub-pipelines that each return all documents and verify they see exactly the same $rand values