Add update array tests for $push

[vic-tsang]

This PR contains:

• update array tests for $push

Ref:

• Issue Add compatibility test for $push (update array) #386

[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: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (471+0 LOC, 7 files); LLM: Adds new compatibility test cases for the $push array update operator, expanding test coverage in the tests directory.

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