test: protect class name compatibility

[gpbl]

Changes proposed for #3015 can break existing consumers by widening public class name outputs to string | undefined or by restoring default classes when an override explicitly supplies undefined. These regression checks preserve the current contracts before changing the input prop type.

What Changed

• Add compile-time assertions that getDefaultClassNames().root and useDayPicker().classNames.root remain assignable to string.
• Add a component test confirming that classNames={{ root: undefined }} removes the default root class.
• Keep production code unchanged; this PR does not fix Types: allow undefined in ClassNames values (classNames breaks under exactOptionalPropertyTypes) #3015.

Validation: both affected Jest suites pass (58 tests), workspace typechecking passes, and Biome passes for the changed files. Mutation checks confirm the new component test rejects filtering out undefined overrides, and the new type assertions reject widening ClassNames values to string | undefined.

[changeset-bot]

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Latest commit: 638fe0f

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