server: enhance FIFO prompt cache eviction with second-chance algorithm

[nonml]

• Pure FIFO eviction always removes the oldest cache entry
• A small side session request evicts a large session's cached KV state immediately, forcing full reprocessing and preventing users on small machines from switching sessions back and forth.

What the fix does:

• Assign score = 1 to new cache entries
• Only on a cache hit (best match) score += 1 (max score=4) -> 4 is the sweet spot
• During checkpoint limit reached, instead of always evicting first out,
  • If front score <= 1, evict it
  • Otherwise, score -= 1 rotate it to the back (second chance), repeat
• If all entries have max score, cap max iter at states.size() * 5 and falls back to evicting the front to prevents infinite loops

With this algorithm, I can switch between sessions without sacrificing compute.

Additional information

related issue (#23030, #20510)

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Yes - researching for solution

[jacekpoplawski]

I initially tried this #22826