server: save and clear idle slots on new task (`--clear-idle`) (#20993)

server: save and clear idle slots on new task (`--clear-idle`) (#20993)

* server: clear idle slots KV from VRAM (LLAMA_KV_KEEP_ONLY_ACTIVE)

* server: move idle slot KV clearing to slot release

The save "cost" is now paid by the finishing request.

* server: add --kv-clear-idle flag, enable by default

* server: skip clearing last idle slot, clear on launch

* server: test --no-kv-clear-idle flag

* server: simplify on-release clearing loop

* server: remove on-release KV clearing, keep launch-only

* cont : clean-up

* tests: update log strings after --clear-idle rename

* tests: use debug tags instead of log message matching

* test: fix Windows CI by dropping temp log file unlink

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Co-authored-by: Georgi Gerganov <redacted>