fix(stepfunctions): prefixes not appended to states in parallel branches

[kaizencc]

There is an order mismatch issue when calling Parallel.branch(). branch used to immediately create the state graph of the branch which locks in the stateIds of the branch states. This fails to take into account the following scenario:

class ParallelMachineFragment extends stepfunctions.StateMachineFragment {
  public readonly startState: stepfunctions.State;
  public readonly endStates: stepfunctions.INextable[];

  constructor(scope: Construct, id: string) {
    super(scope, id);

    const step1 = new stepfunctions.Pass(this, 'Step 1');
    const parallelState = new stepfunctions.Parallel(this, 'Parallel State');
    const chain = parallelState.branch(step1);
    this.startState = parallelState;
    this.endStates = [chain];
  }
}

class MyStack extends Stack {
  constructor(scope: Construct, id: string) {
    const fragment1 = new ParallelMachineFragment(this, 'Fragment 1').prefixStates();
    const fragment2 = new ParallelMachineFragment(this, 'Fragment 2').prefixStates();
    
    new stepfunctions.StateMachine(stack, 'State Machine', {
      definition: fragment1.next(fragment2),
    });
  }
}

Here, the branches in the parallelState do not get prefixes before they are evaluated, causing an error since they have the same stateId.

The fix is to make the state graph creation late-bound; it now happens when we call Parallel.bindToGraph(). At this point, it will know of any prefixes.

Fixes #17354.

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[gitpod-io]

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