Add $convertToCapped command tests

[danielfrankcom]

This change adds tests for the $convertToCapped collection command.

As with similar changes, the intent is to test the input/output specs, not the underlying engine functionality for this operator.

Ref: #26

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort XL · Status Needs Review
Confidence: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (2593+5 LOC, 11 files); LLM: Adds new test cases for the $convertToCapped command to the compatibility test suite, a straightforward coverage expansion with no schema or cross-component changes.

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[SaiPradyumn]

readConcern behavior: Standard parameter tested in every other command PR.

[eerxuan]

FYI, has merge conflict.