Reject negative cache size limits

[CodingFeng101]

Summary

Reject negative CACHE_SIZE_LIMIT values during settings validation. The setting is used as a cache-size limit, but negative integers are currently accepted and make the cache eviction condition true immediately after entries are added.

Changes

• Add a non-negative validation check for CACHE_SIZE_LIMIT.
• Add a focused regression test for negative values.
• Keep the existing behavior for 0 and positive limits unchanged.

Validation

• python -m pytest tests/test_cache_size_limit.py -q -> 1 passed
• Manual smoke check confirmed CACHE_SIZE_LIMIT values 0 and 1 are still accepted.