Add $convert tests

[danielfrankcom]

This change adds $convert tests.

Covers:

• Target type identification (string names, numeric codes, object form)
• binData conversions and subtype handling
• format and byteOrder parameters
• onError and onNull behavior
• Null/missing input and validation skipping
• Conversion errors, invalid arguments, and size limits
• Expression arguments and document field references

The $convert operator is the general form behind $toString, $toInt, $toDouble, etc. The various $to* tests are already parametrized over both the native operator and the equivalent $convert expression, so the test coverage is completely identical and there is no need to duplicate those test cases. The tests included here are unique to $convert, covering options that are not applicable to the $to* operators.

Note: there is a possible MongoDB bug where converting a Date with year outside 0-9999 to string produces error code 18537 (DateToStringYearRange) that is not caught by onError, unlike all other conversion errors. The date_str_year_err_on_error_caught_* tests document the correct behavior where onError handles this case, and are expected to fail against a server with this bug.

[danielfrankcom]

The 2 remaining CI failures are the expected date_str_year_err_on_error_caught_* failures.

Once #79 is merged I can mark these as expected failures for MongoDB, but I don't want to duplicate the framework effort for adding that machinery.