test(e2e): anchor the phase-warning guard on its own unique phrase (#3897)

The guard in the CPU throttle suite finds the missing-timeout phase
warning by the first match of a phrase that stopped being unique when
the bracket-pair captures started writing their own [bounds] notes with
the same words. The first match is now a note, the guard reads it as the
warning, and the unit gate fails on every run, blocking all PRs. Same
fix the node-join copy of this guard already carries: anchor on the
phrase unique to the warning itself. All fifteen suites that read the
shared library as text pass locally with the change.

hack/run-kubernetes-cpu-throttle_test.bats

@@ -159,7 +159,10 @@ run_capture() {
   # sentence sends the reader to the opposite conclusion about the one collector
   # whose behaviour differs, and a comment in the source does not reach them.
   lib=hack/e2e-chainsaw/_lib/run-kubernetes.sh
-  warn=$(grep -n 'timeout is not on PATH' "$lib" | head -n 1 | cut -d: -f1)
+  # Anchored on the phrase unique to the phase warning itself: the bracket-pair
+  # captures write their own [bounds] notes carrying "timeout is not on PATH",
+  # so the first match of that phrase is no longer this sentence.
+  warn=$(grep -n 'run UNBOUNDED' "$lib" | head -n 1 | cut -d: -f1)
   if [ -z "$warn" ]; then
     echo "expected the phase to still warn when timeout is missing" >&2
     return 1