[P/D] Log warnings related to prefill KV expiry

[njhill]

#20139 added a safety-net for releasing prefill blocks that weren't retrieved or notified by a decode worker within a time window (default two minutes).

This PR adds some warning logs both when such expiries occur and when notifications for unrecognized request ids are received.

Also fixes a small leak for the TP case where we don't also flush consumer_notification_counts_by_req for expired requests.

cc @NickLucche

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[gemini-code-assist]

Code Review

This pull request adds important warning logs for prefill KV cache expiry and notifications for unrecognized requests, and fixes a small memory leak. I've identified a couple of critical issues that could lead to runtime errors and another memory leak. My review provides suggestions to address these.

[NickLucche]

Thanks for spotting this @njhill !
I think there's a couple %d gemini wanted you to replace :)