Addon Vitest: Set screenshotFailures to false by default

[yannbf]

Closes #28904

What I did

The Vitest plugin will set test.browser.screenshotFailures to false by default if not set by the user.

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

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

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>

name	before	after	diff	z	%
createSize	0 B	0 B	0 B	-	-
generateSize	76.3 MB	76.3 MB	0 B	0.57	0%
initSize	169 MB	169 MB	0 B	1.24	0%
diffSize	92.6 MB	92.6 MB	0 B	1.24	0%
buildSize	7.46 MB	7.46 MB	0 B	1.23	0%
buildSbAddonsSize	1.61 MB	1.61 MB	0 B	1.28	0%
buildSbCommonSize	195 kB	195 kB	0 B	-	0%
buildSbManagerSize	2.3 MB	2.3 MB	0 B	1.21	0%
buildSbPreviewSize	351 kB	351 kB	0 B	-0.46	0%
buildStaticSize	0 B	0 B	0 B	-	-
buildPrebuildSize	4.46 MB	4.46 MB	0 B	1.23	0%
buildPreviewSize	3 MB	3 MB	0 B	1.22	0%
testBuildSize	0 B	0 B	0 B	-	-
testBuildSbAddonsSize	0 B	0 B	0 B	-	-
testBuildSbCommonSize	0 B	0 B	0 B	-	-
testBuildSbManagerSize	0 B	0 B	0 B	-	-
testBuildSbPreviewSize	0 B	0 B	0 B	-	-
testBuildStaticSize	0 B	0 B	0 B	-	-
testBuildPrebuildSize	0 B	0 B	0 B	-	-
testBuildPreviewSize	0 B	0 B	0 B	-	-

name	before	after	diff	z	%
createTime	18.7s	6.5s	-12s -188ms	-1.29	🔰-185%
generateTime	20.8s	20.4s	-438ms	0.05	-2.1%
initTime	19.3s	17s	-2s -273ms	-0.33	-13.3%
buildTime	11.4s	12.4s	983ms	0.09	7.9%
testBuildTime	0ms	0ms	0ms	-	-
devPreviewResponsive	8.2s	6.2s	-2s -42ms	-1.5	🔰-32.9%
devManagerResponsive	5.4s	4.1s	-1s -280ms	-1.41	🔰-30.6%
devManagerHeaderVisible	816ms	768ms	-48ms	-0.58	-6.2%
devManagerIndexVisible	863ms	823ms	-40ms	-0.36	-4.9%
devStoryVisibleUncached	1.5s	1s	-537ms	-1.65	🔰-50.6%
devStoryVisible	875ms	824ms	-51ms	-0.47	-6.2%
devAutodocsVisible	684ms	799ms	115ms	0.54	14.4%
devMDXVisible	695ms	662ms	-33ms	-0.77	-5%
buildManagerHeaderVisible	857ms	629ms	-228ms	-1.46	🔰-36.2%
buildManagerIndexVisible	866ms	635ms	-231ms	-1.45	🔰-36.4%
buildStoryVisible	963ms	666ms	-297ms	-1.52	🔰-44.6%
buildAutodocsVisible	774ms	635ms	-139ms	-0.83	-21.9%
buildMDXVisible	723ms	558ms	-165ms	-1.56	🔰-29.6%

Greptile Summary

This PR sets the default behavior for screenshot failures in Storybook's Vitest plugin, addressing issue #28904.

• Modified code/addons/vitest/src/plugin/index.ts to set screenshotFailures to false by default in browser tests
• Added a conditional check to only set the default if config.test.browser exists
• Implemented using optional chaining (??=) for concise code and to preserve user-defined values

[greptile-apps]

LGTM

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