Add $avg accumulator tests

[alinaliBQ]

This change adds tests for the $avg accumulator operator.

Add accumulator operator tests for $avg. Tests database $avg behavior, output collection, syntax, and expected errors.
Integration tests are in documentdb_tests/compatibility/tests/core/operator/accumulators/test_accumulators_avg_integration.py

• Add compatibility test for accumulator operators (11 total) #28

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort XL · Status Needs Review
Confidence: 0.92 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (2914+0 LOC, 14 files); LLM: Adds new integration test coverage for the $avg accumulator operator under the compatibility tests path, a meaningful but non-blocking functional improvement.

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[alinaliBQ]

I will address applicable comments for #214 in this PR as well.

[alinaliBQ]

Removed stage tests and renamed files to test_accumulator_*.py

[eerxuan]

1. Lost coverage: $group arity tests. The 3 arity tests for $avg in $group ({"$avg": ["$v", "$v"]} →
   expect GROUP_ACCUMULATOR_ARRAY_ARGUMENT_ERROR) were dropped along with the $bucket/$bucketAuto versions.
   The $group ones are genuinely accumulator-contract tests — they're verifying that $avg rejects
   multi-element-array form regardless of stage. Suggest re-adding just the $group cases:
   AccumulatorTestCase("arity_multi_element_group",
   pipeline=[{"$group": {"_id": None, "result": {"$avg": ["$v", "$v"]}}}],
   error_code=GROUP_ACCUMULATOR_ARRAY_ARGUMENT_ERROR,
   msg="$avg should reject multi-element array syntax in $group"),
   AccumulatorTestCase("arity_empty_array_group",
   pipeline=[{"$group": {"_id": None, "result": {"$avg": []}}}],
   error_code=GROUP_ACCUMULATOR_ARRAY_ARGUMENT_ERROR,
   msg="$avg should reject empty array syntax in $group"),
   AccumulatorTestCase("arity_single_element_group",
   pipeline=[{"$group": {"_id": None, "result": {"$avg": ["$v"]}}}],
   error_code=GROUP_ACCUMULATOR_ARRAY_ARGUMENT_ERROR,
   msg="$avg should reject single-element array syntax in $group"),

[alinaliBQ]

@eerxuan Thanks for reviewing, GROUP_ACCUMULATOR_ARRAY_ARGUMENT_ERROR is covered in

functional-tests/documentdb_tests/compatibility/tests/core/operator/stages/group/test_group_accumulator_errors.py

Line 232 in 2b31be9

GROUP_ACCUMULATOR_ARRAY_ARGUMENT_ERROR_TESTS: list[StageTestCase] = [

I think the $group argument array errors fall under the $group category and not the accumulators.