Add $min accumulator tests

[alinaliBQ]

This change adds tests for the $min accumulator operator.

Add accumulator operator tests for $min. Tests database $min behavior, output collection, syntax, and expected errors.
Integration tests are in documentdb_tests/compatibility/tests/core/operator/accumulators/test_accumulators_min_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 (2919+2 LOC, 12 files); LLM: Adds new integration test coverage for the $min accumulator operator under the compatibility tests path, a meaningful but self-contained test-coverage contribution.

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[alinaliBQ]

Removed stage tests and renamed tests to test_accumulator_*.py

[eerxuan]

Please apply the same feedback on #214 (review)

[alinaliBQ]

applied #214 (review): add arity tests, expression tests, BSON tests, order dependence tests, and empty-group behavior tests. Remove BSON order tests.