fix: compare resize target against current allocator capacity

[jeff3071]

Summary

Fix the resize watcher failing to restore an allocator to its initial capacity after shrinking.

mem_size_per_layer_ stores the startup capacity and is not updated by resize(). The watcher therefore treated the initial limit as unchanged even when the allocator was currently smaller.

Use num_total_pages_ * page_size_ as the current allocator capacity instead.

Test

Added a regression test covering:

1. Shrink from the initial capacity.
2. Restore the original limit.
3. Verify the watcher produces the expansion target.
4. Verify the allocator returns to its initial page count.

python -m pytest -v  tests/test_kvcache_manager.py::test_resize_watcher_can_restore_initial_capacity