Add $toString, $toBool, $toObjectId tests

[PatersonProjects]

This PR adds the $toString, $toBool, and $toObjectId tests. It was originally authored by @danielfrankcom

Ref: Issue #15, #331, #325, #330

[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); effort from diff stats (3041+0 LOC, 25 files); LLM: Adds new compatibility test cases for $toString, $toBool, and $toObjectId operators, expanding test coverage across multiple related issues.

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[danielfrankcom]

Sorry I missed this before, but all of the $toX operators also need to be parameterized against $convert: {input, to} since they are an alias, and the $convert coverage relies on them existing.

See this comment.

[danielfrankcom]

I think every test in these files was originally parameterized over $convert too. These $toX operators are essentially an alias for $convert: {input, to}.

I missed this in the context of #684 but we should probably fix it there too as it's a significant coverage gap. The $convert tests rely on these split coverage.

[danielfrankcom]

I think we need an equivalent case for the other 2 operators, this seems to be the only one.