fix(types): allow undefined in ClassNames values

[cpruijsen]

Summary

ClassNames values are string | undefined, so Partial<ClassNames> accepts string | undefined from clsx / cva / similar helpers under TypeScript exactOptionalPropertyTypes (TS2375). Keys stay required; they are not made optional.

The public API change is types-only. Internally, values are narrowed before classList / string[], which is what tsc requires after that change.

Fixes #3015.

Decision

Chose ClassNames values string | undefined, matching Styles and the change proposed in the issue.

The alternative is to keep ClassNames as string and only widen the classNames prop to { [K in keyof ClassNames]?: string | undefined }, so getDefaultClassNames() / useDayPicker().classNames stay definitely string and the animation / modifier helpers need no edits.

Happy to switch to the prop-only widen.

Test plan

• pnpm --filter react-day-picker typecheck (fails without the ClassNames value change; passes with it)
• Isolated tsc with exactOptionalPropertyTypes: true assigning { root: string | undefined } to Partial<ClassNames>
• jest --selectProjects src --testPathPatterns='types/props.test|useAnimation.test|getDefaultClassNames.test'
• If reviewing locally: pnpm test and pnpm typecheck as in CONTRIBUTING.md

[changeset-bot]

🦋 Changeset detected

Latest commit: f157090

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
react-day-picker	Patch
@daypicker/react	Patch
@daypicker/buddhist	Patch
@daypicker/ethiopic	Patch
@daypicker/hebrew	Patch
@daypicker/hijri	Patch
@daypicker/persian	Patch

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