workflow: add hook conflict inspection

[fantix]

This PR adds support for a workflow to check whether a hook token is already in use without waiting for someone to send data to the hook.

New public API: HookEvent.get_conflict() -> Run[Any] | None. Calling it claims the token for this hook if the token is free. It returns None in that case, or the owning Run if another hook already uses the token.

hook = Approval.wait(token=f"order:{order_id}")

conflict = await hook.get_conflict()
if conflict is not None:
    return {"status": "already-running", "run_id": conflict.run_id}

approval = await hook
return {"status": "approved", "by": approval.user_id}

---

• Keep the owner run ID when a hook token conflicts.
• Add HookEvent.get_conflict() to claim a token without waiting for hook data.
• Continue the workflow as soon as that token claim succeeds.
• Return the same owning Run from repeated conflict checks.
• Materialize lazy Vercel hook conflict events so the runtime can inspect them.