fix(types): allow undefined in ClassNames for exactOptionalPropertyTypes

[jafari1995]

Summary

ClassNames typed every slot as string, so with exactOptionalPropertyTypes you couldn't pass string | undefined from helpers like clsx/cva. Styles already allows undefined — this lines ClassNames up with that.

Fixes #3015

Test plan

• Updated the ClassNames value type to string | undefined
• Added a compile-time check in props.test.tsx using string | undefined
• CI typecheck

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Latest commit: 046f42c

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[jafari1995]

Pushed a follow-up for the CI failures:

• Keep `ClassNames` values as `string` (avoids breaking internal `classList` usage)
• Widen the `classNames` prop to allow `string | undefined` under `exactOptionalPropertyTypes`
• Filter out `undefined` when merging with defaults

Local `pnpm typecheck` is green.

[jafari1995]

Closing this. Empty string covers the exactOptionalPropertyTypes case, and I'd rather not change the public types.