Add boundary case tests for scheduler with homogeneous priority queues

[coderabbitai]

Background

This issue is a follow-up to PR #189 which adds parameterized tests for scheduler priority behavior when fairness is disabled.

Problem

The current test cases in tests/connection_actor.rs only test scenarios with mixed priority frames. We need boundary case testing to ensure the scheduler handles edge cases correctly when queues contain only high-priority or only low-priority frames.

Proposed Solution

Add boundary case test scenarios to verify the scheduler handles empty queues correctly when:

1. Only high-priority frames are present
2. Only low-priority frames are present

These tests should verify that the scheduler processes frames correctly without attempting to access empty queues of the opposite priority.

Acceptance Criteria

• Add test case with only high-priority frames (e.g., [High, High, High])
• Add test case with only low-priority frames (e.g., [Low, Low, Low])
• Verify frames are processed in the correct order (should maintain arrival order within same priority)
• Ensure no runtime errors occur when one priority queue is empty
• Tests should pass with the current scheduler implementation
• Follow existing code patterns and use the Priority enum

Context

• Related PR: Add test for strict priority when fairness disabled #189
• Test file: tests/connection_actor.rs
• Function: fairness_disabled_processes_all_high_first (or create new test function if needed)
• Current test focus: Mixed priority scenarios

Implementation Notes

These boundary cases could be added as additional #[case] attributes to the existing parameterized test, or as separate test functions if the logic differs significantly.

Consider edge cases like:

• Empty queues of opposite priority
• Proper iterator handling when one priority type is absent
• Maintaining arrival order within the same priority level

Backlinks:

• PR: Add test for strict priority when fairness disabled #189
• Requested by: @leynos