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.