MachineScheduler cache loses track of bound machines

[lukasfrank]

Proposed Changes

• Fix instance 5ebaf389-19c5-46dc-a108-6a99f18607c4 is not present in the cache and thus cannot be updated error
• Fix missing return nil in ForgetInstance after successful removal.
• Use MaxAllocatable() in fitsPool to reject pools that are full.

Fixes #1491

[coderabbitai]

📝 Walkthrough

Walkthrough

Machine and volume schedulers now fit pools from MaxAllocatable, add newly assigned instances to cache before the normal update path, and use write locks in cache mutation methods. Tests were updated and extended to cover cache-state tracking for both controllers.

Changes

Scheduler cache and assignment updates

Layer / File(s)	Summary
Cache mutation locking internal/controllers/compute/scheduler/cache.go, internal/controllers/storage/scheduler/cache.go	ForgetInstance and FinishBinding now use write locks, and successful forgets return nil after removing assumed instances.
Machine scheduler flow internal/controllers/compute/machine_scheduler.go, internal/controllers/compute/machine_scheduler_test.go	MachineScheduler fits pools from MaxAllocatable, adds newly assigned machines to cache before continuing the update path, and the machine tests remove Image fields while adding cache-state coverage.
Volume scheduler flow internal/controllers/storage/volume_scheduler.go, internal/controllers/storage/volume_scheduler_test.go	VolumeScheduler fits pools from MaxAllocatable, adds newly assigned volumes to cache before continuing the update path, and the volume test checks two assigned volumes and one unscheduled volume.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main change to MachineScheduler cache tracking.
Linked Issues check	✅ Passed	The changes address #1491 by keeping bound instances in the scheduler cache and fixing the related update path.
Out of Scope Changes check	✅ Passed	The volume scheduler and cache updates are still within the linked issue's scope, which covers both machine and volume cache tracking.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The PR description matches the required template, includes three proposed changes, and references the linked issue.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/scheduler-bugs

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[friegger]

For the commit message I would like to see a small description what the impact of e.g. using MaxAllocatable() is.