[Workers] Add changelog for @cloudflare/vitest-pool-workers Vitest 4 support

[MattieTK]

Adds the changelog entry for @cloudflare/vitest-pool-workers v0.13.0 (Vitest 4 support), spun out from #28908 which landed the docs changes separately.

Changes from the version reviewed in #28908:

• Leads with the bug fix story rather than the breaking changes — library import resolution, Node.js specifier resolution, nodejs_compat auto-enabling, removal of the 8 KB provide data limit, Vitest UI fix
• Adds a note that the codemod only covers vitest.config.ts, not test files (addresses irvinebroque and Cherry's comments)
• Restores the isolatedStorage/singleWorker bullet under breaking changes with the --max-workers=1 --no-isolate workaround
• Adds a copyable coding agent prompt covering all migration steps, including a link to the Vitest 4 upstream migration guide for test file changes

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/changelog/	@cloudflare/pm-changelogs, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://9b91e178.preview.developers.cloudflare.com
Preview Branch URL: https://mattietk-vitest-pool-workers-vitest4-changelog.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/changelog/2026-03-11-vitest-pool-workers-vitest-4/	https://mattietk-vitest-pool-workers-vitest4-changelog.preview.developers.cloudflare.com/changelog/2026-03-11-vitest-pool-workers-vitest-4/