[@mantine/dates] MonthsList/YearsList: Fix children override in getMo…

[MehdiAlipooor]

What this PR does

This PR fixes an issue where getMonthControlProps and getYearControlProps did not properly
override the default control children in date picker components.

Fixes:

• children returned from getMonthControlProps in <MonthsList /> are now rendered correctly instead of the default formatted month label.
• children returned from getYearControlProps in <YearsList /> are now rendered correctly instead of the default formatted year label.

Additional changes:

• Added to storybook to ensure custom children override behavior for month controls.
• Ensured consistency between month and year control props behavior.

Why

Consumers expect these props to fully override the rendered content of month/year picker controls.
Previously, the default label still rendered, making customization impossible.

Breaking changes

None.

Checklist

• Fix children override behavior in MonthsList and YearsList component
• Add storybook for custom month control children in MonthsList and YearsList
• Ensure year control props behave consistently
• All existing tests pass

[rtivital]

Thanks!