Add $avg accumulator tests

[alinaliBQ]

This change adds tests for the $avg accumulator operator.

Add stage 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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