fix: incorrect tracking of usable conns in reaper

[quettabit]

closes #128

[greptile-apps]

Greptile Summary

This PR corrects idle-connection reaping by basing the retention threshold on non-retired, available connections rather than every connection in the host pool.

• Prevents unavailable or retired connections from causing the last usable idle connection to be reaped.
• Updates the usable count as idle connections are selected for closure.

Confidence Score: 5/5

The PR appears safe to merge; the revised accounting consistently preserves one usable connection while pruning stale pool entries.

The reaper computes and updates its usable-connection count synchronously, and no changed behavior was found that introduces a concrete correctness, compatibility, or security failure.

Important Files Changed

Filename	Overview
src/s2_sdk/_client.py	Corrects reaper accounting so idle cleanup retains one available, non-retired connection per host.

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