Fix parallel smoke test: stub do_action for the run-completed hook

workflow-parallel-smoke.php defines its own inline WordPress stubs and had
add_filter/apply_filters/add_action but not do_action. The runner now fires
do_action( 'wp_agent_workflow_run_completed', ... ) at the terminal funnel, so
run() under this stub set hit "Call to undefined function do_action()" and the
test fataled (exit 255). Add the missing do_action stub — it dispatches the
callbacks registered via the add_action/add_filter stubs — so the completion
hook fires in the test the same way it does under WordPress. Production code is
unaffected; only the test harness lacked the stub.

## AI assistance
- **AI assistance:** Yes
- **Tool(s):** Claude Opus 4.8 via Claude Code
- **Used for:** Diagnosing the missing-stub CI failure and adding the do_action stub.

Author: chubes4

tests/workflow-parallel-smoke.php

@@ -87,6 +87,17 @@ function add_action( string $hook, callable $cb, int $priority = 10, int $accept
 		add_filter( $hook, $cb, $priority, $accepted_args );
 	}
 }
+if ( ! function_exists( 'do_action' ) ) {
+	function do_action( string $hook, ...$args ): void {
+		$cbs = $GLOBALS['__filters'][ $hook ] ?? array();
+		ksort( $cbs );
+		foreach ( $cbs as $bucket ) {
+			foreach ( $bucket as $cb ) {
+				call_user_func_array( $cb, $args );
+			}
+		}
+	}
+}
 if ( ! function_exists( 'wp_get_ability' ) ) {
 	function wp_get_ability( string $name ) {
 		return $GLOBALS['__abilities'][ $name ] ?? null;