Add workflow scoped drain await helper

[chubes4]

Summary

• Add a generic WP_Agent_Workflow_Scoped_Drain::drain_suspended_run() helper for foreground await/status callers.
• Reuses the existing scoped Action Scheduler drain while centralizing the suspended-run terminal callback.
• Keeps product code responsible for choosing a safe foreground context.

Root cause

Fanout consumers had a generic scoped drain primitive, but no generic helper that binds a suspended workflow run plus recorder to that drain. Callers had to wire branch/resume hook scope and terminal-state polling themselves, which left status/await paths easy to miss on runtimes where no WP-Cron runner pumps Action Scheduler.

Verification

• php -l src/Workflows/class-wp-agent-workflow-scoped-drain.php
• php tests/workflow-scoped-drain-smoke.php
• php tests/workflow-parallel-async-smoke.php

Related

• Supports chubes4/wp-build#1028 / chubes4/wp-build#962 by providing the upstream generic drain contract.

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via opencode
• Used for: Diagnosed the fanout drain ownership layer, implemented the helper, and ran focused verification.