[Bug] AddressManager accepts size 0, breaks free list / ZeroDivisionError

Hey team,

I found `AddressManager.allocate()` and `batched_allocate()` say size should be > 0 but don't check it. On dev (68b7e5f):

```python
from lmcache.v1.memory_management import AddressManager

am = AddressManager(1 << 20)
a = am.allocate(4096)
z = am.allocate(0)
b = am.allocate(4096)
am.free(*z); am.free(*b); am.free(*a)
print([(x.start, x.size) for x in am._explicit_list])
# [(0, 4096), (4096, 4096), (8192, 1040384)] <- never coalesced
am.allocate(1 << 20)
# RuntimeError: no memory is available (everything is free)

AddressManager(1 << 20).batched_allocate(0, 3)
# ZeroDivisionError (block.size // aligned_size)
```

The ZeroDivisionError also gets past `TensorMemoryAllocator.batched_allocate`, which only catches RuntimeError.

I hit this through `SegmentTokenDatabase`: a leading/trailing/double separator gives an empty range (e.g. `(4, 4)`), and `LMCacheEngine.store()` allocates 0 bytes for it.

Fix I'd suggest: raise RuntimeError for size <= 0 in both (same as #5061 did for batch_size) and add tests in `tests/v1/test_address_manager.py`. Happy to send a PR.

Start with AddressManager.allocate() and batched_allocate() in lmcache/v1/memory_management, then read the related tests in tests/v1/test_address_manager.py. Add coverage for zero and negative sizes, including batched allocation, and verify invalid sizes raise RuntimeError without corrupting the free list or causing ZeroDivisionError.

Written by the indexing model from the issue text.