Viewport: Skip viewport validation before parameters load

[ia319]

Closes #33777

What I did

Skip viewport validation when parameter is not yet available to prevent premature resets

Why

initialGlobals.viewport.value is validated before INITIAL_VIEWPORTS is loaded, causing valid viewports to be treated as invalid and reset.

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

Add the configuration to preview.ts:

import { INITIAL_VIEWPORTS } from "storybook/viewport";

const preview: Preview = {
  parameters: {
    viewport: {
      options: INITIAL_VIEWPORTS,
    },
  },
  initialGlobals: {
    viewport: {
      value: "iphone14",
      isRotated: true,
    },
  },
};

PS

If initialGlobals.viewport.value is not present in parameters.viewport.options, it is silently reset without warning or error.

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-storybook/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 publish.yml --field pr=<PR_NUMBER>

Summary by CodeRabbit

• Bug Fixes
  • Resolved an issue where viewport configuration could be applied before required parameters finished loading, ensuring stable initialization behavior.

[coderabbitai]

📝 Walkthrough

Walkthrough

Updates a viewport hook to prevent processing when a parameter has not yet loaded, adding a guard condition and extending the dependency array to avoid race conditions with default values.

Changes

Cohort / File(s)	Summary
Parameter Loading Guard code/core/src/viewport/useViewport.ts	Adds early return and dependency array extension to prevent processing before parameter is loaded, avoiding race conditions with default MINIMAL_VIEWPORTS.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

✨ Finishing touches

• 📝 Generate docstrings

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