Manual port of @jethac's #63522 onto the pruned tree: 3 of the original
6 fixed-budget poll sites survive (3-concurrent-agents wait, session-A/B
routing wait, two-session queue wait). Replaced each 2.5-5s hard-ceiling
poll loop with the PR's _wait_until(predicate) helper — generous 30s
ceiling reached only on genuine failure, instant return on the green
path, assert with message instead of silent fall-through.
Dropped: the 3 hunks targeting pruned code, and the unrelated
scripts/release.py mailmap hunk (AUTHOR_MAP is frozen; contributor
mapping handled via contributors/emails/).
