refactor: implement ImageRequestHandler class

[drduhe]

Issue #, if available: n/a

Notes

This PR introduces refactoring of the ImageRequestHandler class to decouple region processing logic from ModelRunner. The ImageRequestHandler now makes it the image processing logic self-contained, modular, and easier to test. Additionally, changes to the ModelRunner class were made to initialize this handler with the appropriate dependencies. Methods were refined in the ImageRequestHandler class to make them more compact and testable.

Unit tests were updated to split out region processing testing associated with the new class.

Additional method was added to the JobItem dataclass which allows conversion to from an ImageRequest to a JobItem.

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