bc58aa1be8 made _manual_compress_turn raise CompressionBusy instead of a bare
RuntimeError, so /compress and the slash mirror answer a lost race with
their busy reply instead of "compress failed". No test reached the
_compress_live_with_feedback path that /compress and the slash mirror share.
While the compaction holds the claim, the existing test now calls it and
expects CompressionBusy (RuntimeError before bc58aa1be8).
Gate finding: C04 r2 2ab [Low TOOTHLESS].
Refs #133504
1369085be4