server: enforce prompt cache RAM limit (#25070)
Before this commit, --cache-ram was not a hard limit:
- The cache always kept at least one entry, even if that entry exceeded the
RAM/token limits.
- Old entries were only evicted for the RAM/token limits after saving the new
one, which could cause the cache to temporarily exceed the RAM/token limits
even if individual entries were below the limit.
Now, ensure that the RAM limit is strict with these changes:
- Skip saving state to cache if by itself it exceeds the RAM limit.
- Evict old entries as necessary to make the new entry fit.
Additionally, token-limit cleanup may now evict the last remaining cache entry
instead of always preserving one.
