workflow: fix replay failures for disposed hooks

[fantix]

This PR fixes a bug where disposing a hook can incorrectly cause workflow replay to fail, as in the following example (step and hook definitions omitted):

@workflows.workflow
async def process_order(order_id):
    inventory = asyncio.create_task(check_inventory(order_id))
    order = await load_order(order_id)  # Runs alongside the inventory check.

    approval = Approval.wait(token=order.approval_token)
    if not await inventory:
        approval.dispose()  # Out of stock: approval is no longer needed.
        return await notify_out_of_stock(order_id)

    decision = await approval
    approval.dispose()
    return await apply_decision(order_id, decision)

1. The order loads while the inventory check is still pending. The workflow creates the approval hook, then suspends at await inventory.
2. Before the runtime persists the hook registration, the inventory check records an out-of-stock result.
3. On replay, that result is read first. The workflow calls approval.dispose(), removing the hook from active suspensions.
4. Replay then reaches the hook registration event. It only checks active suspensions (context.suspensions), so it incorrectly reports the hook as missing.

This PR fixes to look up disposed hooks in the hook registry (context.hooks) so their registration events can still be processed.

[msullivan]

Should we put the step-cancellation hooks in hooks instead/also?

[fantix]

Yes! Let me do that in a new PR.