fix: kill leftover stalwart process before starting integration tests

The CI self-hosted runner can leave a stalwart process alive from a prior
run that was interrupted externally, causing the next run to fail with
"port already in use". Kill any existing stalwart before starting a new one.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

stalwart-dev/test.sh

@@ -17,6 +17,10 @@ command -v stalwart >/dev/null || {
     exit 1
 }
 
+# Kill any stalwart left over from a previous run (the CI self-hosted runner
+# keeps processes alive across jobs when a run is killed externally).
+pkill -x stalwart 2>/dev/null && sleep 0.5 || true
+
 # Pre-seed spam-filter version so Stalwart does not fetch it on first boot.
 mkdir -p "$STALWART_TMPDIR"
 sqlite3 "${STALWART_TMPDIR}/data.sqlite" \