fix(migrations): parse the --config flag the usage line documents (#18817)

tools/scripts/run_migrations.sh

@@ -15,7 +15,30 @@
 set -e
 
 PY="${PY:-python3}"
-CONFIG="${1:-conf/service_conf.yaml}"
+CONFIG="conf/service_conf.yaml"
+
+while [ $# -gt 0 ]; do
+    case "$1" in
+        --config)
+            # Require a real file. mysql_migration.py:90 turns any config-load
+            # failure into a warning and falls back to host=localhost user=root
+            # database=rag_flow, and this script also passes --execute, so an
+            # empty, whitespace, flag-shaped or missing path would silently
+            # migrate the default database instead of failing.
+            if [ $# -lt 2 ] || [ -z "${2//[[:space:]]/}" ] || [ ! -f "$2" ] || [ "${2#-}" != "$2" ]; then
+                echo "Error: --config requires the path of an existing file" >&2
+                exit 1
+            fi
+            CONFIG="$2"
+            shift 2
+            ;;
+        *)
+            echo "Error: unknown argument: $1" >&2
+            echo "Usage: PY=python3 tools/scripts/run_migrations.sh [--config CONFIG_PATH]" >&2
+            exit 1
+            ;;
+    esac
+done
 
 echo "Running model provider table migrations..."