Add test for scheduler behavior when fairness is disabled (max_high_before_low = 0, time_slice = None)

[sourcery-ai]

There is currently no test that verifies the behavior when fairness is disabled in the scheduler (i.e., when max_high_before_low = 0 and time_slice = None). According to the documentation, in this configuration, all high-priority frames should be processed before any low-priority ones.

Action items:

• Add a test case where max_high_before_low = 0 and time_slice = None.
• The test should assert that all high-priority frames are processed before any low-priority frames.
• Ensure the test covers both the order and completeness of processing.

This will help ensure the scheduler's documented behavior is maintained and prevent regressions in the future.

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I created this issue for @leynos from #147 (comment).

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