Fix stale serializer cache IDs after deletion

[longligooo]

Summary

• Remove deleted object identifiers from the forward cache's eviction queue.
• Prevent a later cache insertion from attempting to evict an identifier that no longer exists.
• Add regression coverage for deleting a cached object and filling the freed cache slot.

Related Issues / Discussions

Closes #9370

QA Instructions

Run the focused regression test:

pytest -q tests/test_object_serializer_disk.py::test_obj_serializer_fwd_cache_removes_deleted_ids_from_eviction_queue

Run the configured lint check on the changed files:

ruff check invokeai/app/services/object_serializer/object_serializer_forward_cache.py tests/test_object_serializer_disk.py

Merge Plan

No special merge steps are required.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Changes to a redux slice have a corresponding migration (not applicable)
• Documentation added / updated (if applicable) (not applicable)
• Updated What's New copy (if doing a release after this PR) (not applicable)