IdleTracking uses lists instead of SortedSet

[Scooletz]

This PR changes how IdleTrackingBackgroundService does bookkeeping for the idle sessions. Instead of using a SortedSet that requires a lot of comparisons, it uses two lists for timestamps and sessions ids. Only if the limit is breached, sessions are sorted and picked up to be released. This removes allocations for the SortedSet,Node and .ToArray() and makes the bookkeeping less allocatey.

Motivation and Context

Make IdleTrackingBackgroundService a bit faster and less allocating when performing session bookkeeping.

How Has This Been Tested?

Locally, with the existing suite. If this PR is considered for merging and a component specific test is required I'm happy to provide one.

Breaking Changes

None.

Types of changes

• Optimisation
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed