Svelte: Fix decorators always running twice

[paoloricciuti]

Closes #24549

What I did

Inside PreviewRender.svelte storyFn was called twice, once to get the values and once in the reactive label statement to make it updated whenever storyFn changes. I've added a small utility

// the first time we don't want to call storyFn two times so we just return the values
// we already have from the previous call. If storyFn changes this function will run
// again but this time firstTime will be false
function getStoryFnValue(storyFn){
	if(firstTime){
		firstTime = false;
		return {
			Component,
			props,
			on,
		}
	}
	return storyFn();
}

that the first time return the values already present by the first call to storyFn.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

I tested that decorators only run once. You can verify that by opening one of the svelte sandbox and by going to renderers/svelte/decorators-runs-once verifying that in the console there's only one decorator log.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[paoloricciuti]

tests are failing because they are failing in next too...from what i can see is the react-vite-plugin that get's resolved to an older version that doesn't support vite 5. I've did some basic research and one of the reasons could be that vite 5 deprecated CJS?

[JReinhold]

LGTM! 💪