fix(server): preserve active turns when switching provider instances#22754

fix(server): a busy provider switch fails with its own tagged error

Merge remote-tracking branch 'origin/main' into fix/issue-22748

test(server): hold the queue before a mid-queue provider switch

Merge remote-tracking branch 'origin/main' into fix/issue-22748

fix(server): refuse provider switches during active turns

test(server): takeover tests for active provider instance switches