End an eviction pass that cannot evict anything

[montehurd]

Fixes git-pkgs#341.

A failed delete leaves the record eligible, so the same batch comes back on the next iteration while the recorded size never drops. Against a backend refusing deletes the pass retries forever, burning CPU and flooding the log. A canceled context makes it faster, since every delete fails at once.

The pass now ends when a batch clears no record, and returns as soon as the context is canceled. Progress counts records rather than bytes, because a cached record can have no size recorded. A pass can therefore end with the cache still above its limit, leaving the failed records for the next pass.

Two disclosures. The zero-size test the issue asks for pins the behaviour but is not load-bearing: its sizeless record shares a batch with a sized one, so a bytes-based rule would still pass it. Separately, the storage interface documents Delete as returning nil for a missing path; the file backend honours that but GCS returns the raw error, so there a record whose bytes are already gone can become a permanently unevictable LRU head. That looks worth its own fix rather than widening this one.

CI here fails on the pre-existing handler test build, which git-pkgs#362 fixes. This same commit stacked on that fix is green across all six jobs: https://github.com/montehurd/proxy/actions/runs/35656065103