Vite: Merge assetsInclude property with Storybook default values

[yuemori]

What I did

In this PR, I address a configuration issue in builder-vite where the assetsInclude field was being statically set to ['/sb-preview/**'], which could potentially overwrite any user-defined configurations. The updated implementation now merges any existing entries with these required defaults, ensuring both are preserved.

Details of Changes

• Modified how assetsInclude is initialized, ensuring it now merges ['/sb-preview/**'] with any existing patterns set by the user.
• Implemented this using spread syntax to efficiently combine arrays, accommodating both singular and multiple existing entries.

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

Documentation

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

Workaround

You can avoid this issue by using viteFinal to further override the settings.

const config: StorybookConfig = {
  // ...,
  viteFinal(config: InlineConfig) {
    return mergeConfig(config, {
      assetsInclude: ["./src/**/*.csv"]
    })
  }
}

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>

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