Add $first accumulator tests

[alinaliBQ]

This change adds tests for the $first accumulator operator.

Add accumulator operator tests for $first. Tests database $first behavior, output collection, syntax, and expected errors.
Integration tests are in documentdb_tests/compatibility/tests/core/operator/accumulators/test_accumulators_first_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 L · Status Needs Review
Confidence: 0.92 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (926+2 LOC, 6 files); LLM: Adds new integration test coverage for the $first accumulator operator under the compatibility tests path, a functional improvement to test quality.

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[alinaliBQ]

applied #214 (review): Add tests: arity tests, BSON constant tests, expression tests, expression error propagation, empty-group behavior, and order dependence tests