Change MaxParallelTestModulesOption to int

[mariam-abdulla]

This pull request refactors the handling of the --max-parallel-test-modules command-line option to improve type safety and simplify parsing logic. The most important changes are grouped below:

Command-line option type update:

• Changed the type of MaxParallelTestModulesOption from Option<string> to Option<int> in TestingPlatformOptions, ensuring that the option value is parsed as an integer directly.

Parsing logic simplification:

• Updated the GetDegreeOfParallelism method in TestingPlatformCommand.cs to remove manual string-to-int parsing, leveraging the new integer type for MaxParallelTestModulesOption. This also makes the logic clearer and less error-prone.

Fixes #50526