Add $mergeObjects accumulator tests

[alinaliBQ]

This change adds tests for the $mergeObjects accumulator operator.

Add accumulator operator tests for $mergeObjects. Tests database $mergeObjects behavior, output collection, syntax, and expected errors.
Integration tests are in documentdb_tests/compatibility/tests/core/operator/accumulators/test_accumulators_ mergeObjects_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 In Progress
Confidence: 0.92 (mixed)

Reasoning

component from path globs (test-coverage); effort from diff stats (1642+0 LOC, 8 files); LLM: Adds new integration test coverage for the $mergeObjects accumulator operator under the compatibility tests path, expanding functional test surface area.

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