Add $maxN array expression compatibility tests (#200)

[hehehe-hahaha]

What does this PR do?

Adds a compatibility test file for the $maxN array expression operator covering 8 edge
case scenarios that were missing from the existing smoke test. The new file tests:

• n larger than array size
• n equal to 1
• Duplicate values in the array
• Negative numbers
• Empty array input
• Null values in the array (verified they are ignored and don't count toward n)
• n as a dynamic field reference per document
• $maxN used as an accumulator inside $group

Why was this PR needed?

The $maxN operator only had one basic smoke test covering a single happy-path scenario.
A compatibility test suite needs to prove the operator works correctly across all realistic
situations, especially edge cases that could silently return wrong results in production.

What are the relevant issue numbers?

Closes #200

Does this PR meet the acceptance criteria?

• Tests added for new/changed behavior
• All tests passing
• Follows project style guide
• No breaking changes introduced

[hehehe-hahaha]

Hi @eerxuan — this is my first contribution to this project. I've added compatibility tests for the $maxN array expression operator as described in issue #200. All 8 tests are passing locally against MongoDB 7.0. Would appreciate a review when you have time!

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P3 · Effort M · Status Needs Review
Confidence: 0.90 (mixed)

Reasoning

component from path globs (test-coverage); effort from diff stats (174+0 LOC, 1 files); LLM: Adds new compatibility test cases for the $maxN array expression operator edge cases, which is a non-blocking test coverage improvement with no functional or build changes.

If a label is wrong, remove it manually and ping @patty-chow so the rules can be tuned. The bot will not re-label items that already have component labels.

[hehehe-hahaha]

Wanted to follow up on this PR. Happy to incorporate any feedback or adjustments. Let me know if there's anything blocking the review and I'll do my best to help move it along!