Add $pop tests

[PatersonProjects]

This PR adds the tests for the $pop array update operator.

Ref: Issue #36, #381

[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 (603+1 LOC, 7 files); LLM: Adds new test cases for the $pop array update operator, expanding compatibility test coverage.

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[eerxuan]

[Minor] pytest_params(TESTS + UPSERT_TESTS) mixes general-behavior and upsert cases through one runner that branches on test.upsert. Splitting into two parametrize calls (or two named runners) would give clearer test IDs and let upsert-specific invariants be asserted without per-case branching.

[eerxuan]

[Minor] test_pop_combination duplicates the body of test_positional_query_operators above (insert → run update → find → assert). Since POP_COMBINATION_TESTS is the same list[UpdateTestCase] shape, consider merging into a single parametrize+runner or extracting a small helper to avoid the copy.