Core: Move prepareContext into store.getStoryContext()

[tmeasday]

This way any consumer of the context will get the properly mapped args.

Closes #22099

What I did

The issue was that although the source decorator emitted the mapped/conditional args, references to store.getStoryContext().args would return the unmapped args.

This is due to #20755 which moved the mapping to before the decorator chain.

This PR moves the mapping slightly earlier, so the context.args is mapped everywhere (not just inside decorators/render). That will make them inconsistent with context.initialArgs, but I think that's probably OK? I'm not sure, this might be considered to risky a change.

In which case we could consider just calling prepareContext in the Source block.

FTR, this inconsistency between context.args + context.initialArgs is already the case inside the decorator stack/render function, so this would only affect outside callers of store.getStoryContext(), which I'm not sure there are any.

How to test

1. Run react sandbox
2. Add this file:

import { Meta, StoryObj } from '@storybook/react';
import React from 'react';

const Example = (props: any) => {
  return <div>Message: {props.hasMessage ? props.message : 'no text'}</div>;
};

type Component = typeof Example;

const meta: Meta<Component> = {
  component: Example,
  tags: ['autodocs'],
  argTypes: {
    hasMessage: { type: 'boolean' },
    message: {
      control: 'text',
      if: { arg: 'hasMessage' },
    },
  },
};

export default meta;

export const Default: StoryObj<Component> = {
  args: {
    hasMessage: false,
    message: 'foo',
  },
};

3. Check the source:

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[tmeasday]

This functionality moved to the store so not really much point unit testing it here. We have a unit test in the story and an integration test in PreviewWeb.test.ts

[tmeasday]

@shilman I ended up not adding using the new CSF type as, until we do #22228, we don't really have the correct type for unmappedArgs.

We may as well leave it as any (as a undefined key on the context) and sort of "undocumented" for now.

[JReinhold]

LGTM! 🥳