fleetd #517: pin the drain-gate abort branch and jar_id's absent case

Two mutation-testing survivors in scripts/redeploy-fleetd.sh: a source-text
test pins what a message SAYS but never whether the branch that prints it is
REACHED.

- Extract the drain-gate abort decision into drain_gate_refusal(do_build,
  staged_path), a pure function the suite can call directly for all four
  build/staged combinations. The existing source-text grep test is kept
  alongside it (it catches a re-wording; the new tests catch a dead branch).
- Extend jar_id's test to cover the missing-file path (both the no-argument
  default and an explicit path), which the #511 test never exercised.

scripts/redeploy-fleetd.sh

@@ -452,6 +452,35 @@ classify_amqp_connection_errors() {
   REDEPLOY_UNEXPLAINED_ERRORS=$((REDEPLOY_UNEXPLAINED_ERRORS + pending_inbox + pending_lead_mailbox))
 }
 
+# fleetd #517: extracted so the suite can call this decision directly, the same way #510 extracted
+# wait_for_daemon_exit so its ordering became checkable. Before this, the only test of the drain-gate
+# abort message was a grep of this script's own source for the wording — so mutating the `if` below
+# to `if false` (making the branch unreachable) left every test green, because the wording was still
+# sitting in the file. Pure: only decides which message applies and prints it, no side effects, so a
+# test can call it directly with an in-memory staged path instead of driving the real drain-gate flow
+# (which needs a live $OLD_PID and an interactive prompt neither test can supply).
+#
+# The four cases:
+#   build ran,  staged jar present -> names the staged jar and how to finish or discard it
+#   build ran,  staged jar absent  -> "nothing changed" (nothing was staged this run either)
+#   --no-build, staged jar present -> ALSO "nothing changed", deliberately: --no-build itself builds
+#     and stages nothing (see require_no_build_jar above), so a staged jar found here is a leftover
+#     from an earlier, unrelated run. THIS run truly changed nothing, and the next DO_BUILD=1 run
+#     wipes that leftover before it builds (`rm -f "$JAR_STAGED"` in the build section above) — so
+#     there is nothing here for the operator to lose track of.
+#   --no-build, staged jar absent  -> "nothing changed"
+drain_gate_refusal() {
+  local do_build="$1" staged_path="$2"
+  if [ "$do_build" = 1 ] && [ -f "$staged_path" ]; then
+    printf 'aborted — the running daemon was NOT touched, but the freshly built jar is sitting at
+        %s, not yet swapped into %s. Rerun WITHOUT --no-build to finish the restart —
+        the freshly built jar is no longer at the live path that --no-build requires — or
+        remove %s by hand if you want to discard this build.' "$staged_path" "$JAR" "$staged_path"
+  else
+    printf 'aborted — nothing changed'
+  fi
+}
+
 # CB-600: sourceable for testing. When this file is SOURCED (not executed) it stops here — nothing
 # below runs — so a test harness can `source` it to call check_log_path_matches_plist (or the
 # other pure helpers above) against a throwaway plist fixture without ever reaching the mutating
@@ -610,16 +639,10 @@ if [ -n "$OLD_PID" ] && [ "$ASSUME_YES" = 0 ]; then
   echo
   read -r -p "   Fleet drained? type yes to restart: " reply
   if [ "$reply" != "yes" ]; then
-    # fleetd #493: "nothing changed" would be a lie once a build has run — the freshly built jar
-    # already moved to $JAR_STAGED (stage_built_jar, above), so the live path has one fewer file
-    # than before this run started, even though the running daemon itself was never touched.
-    if [ "$DO_BUILD" = 1 ] && [ -f "$JAR_STAGED" ]; then
-      die "aborted — the running daemon was NOT touched, but the freshly built jar is sitting at
-          $JAR_STAGED, not yet swapped into $JAR. Rerun WITHOUT --no-build to finish the restart —
-          the freshly built jar is no longer at the live path that --no-build requires — or
-          remove $JAR_STAGED by hand if you want to discard this build."
-    fi
-    die "aborted — nothing changed"
+    # fleetd #493 / #517: "nothing changed" would be a lie once a build has run and staged a jar —
+    # see drain_gate_refusal above for the full decision and why each of its four cases reads the
+    # way it does.
+    die "$(drain_gate_refusal "$DO_BUILD" "$JAR_STAGED")"
   fi
 fi