server: enforce prompt cache RAM limit (#25070)

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.