Add $minN array element tests for null filtering and n validation

[andavag]

What does this PR do?

Adds second-pass compatibility coverage for the $minN array expression operator.

This PR adds tests for:

• filtering null values from $minN array input
• returning all available non-null values when n is larger than the number of usable values
• rejecting invalid n: 0 input with the expected MongoDB error code

Why was this PR needed?

Issue #201 asks for additional $minN array expression compatibility coverage beyond the existing smoke test. The existing test confirms the basic happy path, but it does not cover edge behavior such as null handling or validation of invalid n values.

These tests help document expected MongoDB-compatible behavior and protect against future regressions.

How did you test it?

Focused tests passed against local MongoDB:

python -m pytest documentdb_tests/compatibility/tests/core/operator/expressions/array/minN-array-element/test_minN-array-element_null_filtering.py documentdb_tests/compatibility/tests/core/operator/expressions/array/minN-array-element/test_minN-array-element_n_validation.py --connection-string "mongodb://localhost:27017/?serverSelectionTimeoutMS=2000" --engine-name mongodb

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort M · Status Needs Review
Confidence: 0.30 (deterministic)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (76+0 LOC, 3 files); LLM failed: Invalid response body while trying to fetch https://api.anthropic.com/v1/messages: Premature close

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