Allow -1 (unbounded) parallelism; validate settings

[JimBobSquarePants]

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Description

This pull request enhances the flexibility and robustness of parallel execution in ImageSharp by allowing an unbounded degree of parallelism (using -1), improving validation for parallel execution settings, and updating the parallel row iteration logic accordingly. It also adds comprehensive tests to ensure the new behaviors are correct and validated.

Parallel execution improvements:

• Updated ParallelExecutionSettings to document and support -1 as an unbounded value for MaxDegreeOfParallelism, and clarified this in both the code and XML documentation. [1] [2] [3] [4]
• Refactored the logic in ParallelRowIterator to support unbounded parallelism: introduced GetNumberOfSteps and CreateParallelOptions helpers, and adjusted how the number of steps and parallel options are calculated when MaxDegreeOfParallelism is -1. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Validation and safety enhancements:

• Added ValidateSettings to ensure ParallelExecutionSettings are valid (e.g., MaxDegreeOfParallelism not zero or less than -1, MinimumPixelsProcessedPerTask > 0, and MemoryAllocator not null), guarding against invalid struct defaults and improving error messages.

Testing improvements:

• Added tests to verify that using MaxDegreeOfParallelism = -1 results in all rows being processed, and that default (uninitialized) ParallelExecutionSettings throw appropriate exceptions. Introduced helper types for test operations. [1] [2] [3] [4]