What does the pull request do?

Allows developers to set a custom Width on DatePicker without the content overflowing. Currently, the internal Button has a hardcoded MinWidth from DatePickerThemeMinWidth (296px) that cannot be overridden, preventing compact DatePickers in narrower layouts.

Related discussion: #15456

What is the current behavior?

When setting Width="200" on a DatePicker, the content overflows because the internal Button enforces MinWidth=296 regardless of the control's Width setting.

<DatePicker Width="200" />  <!-- Content overflows -->

What is the updated/expected behavior with this PR?

Developers can now create compact DatePickers by setting MinWidth="0":

<DatePicker Width="200" MinWidth="0" />  <!-- Content fits -->

Default behavior is unchanged - DatePickers without explicit MinWidth still use the theme default (296px).

How was the solution implemented?

1. Added MinWidth and MaxWidth setters to the ControlTheme (preserves default appearance).
2. Removed MinWidth from internal Button, changed MaxWidth to use TemplateBinding.

This allows the Button to shrink when the DatePicker's MinWidth is overridden, while maintaining backward compatibility.

How to test this new behavior:

Add to samples/ControlCatalog/Pages/DateTimePickerPage.xaml:

<TextBlock FontSize="18">A DatePicker with custom Width.</TextBlock>
<StackPanel Orientation="Vertical">
    <Border BorderBrush="Red" BorderThickness="2" Width="200" HorizontalAlignment="Left">
        <DatePicker Width="200" MinWidth="0" />
    </Border>
</StackPanel>

Workaround (for developers waiting for this fix)

Until this PR is merged, you can override the theme resources locally:

<Grid>
    <Grid.Resources>
        <x:Double x:Key="DatePickerThemeMinWidth">0</x:Double>
        <x:Double x:Key="DatePickerThemeMaxWidth">1000</x:Double>
    </Grid.Resources>
    
    <DatePicker Width="200" HorizontalAlignment="Stretch" />
</Grid>

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Fixes #4862
Fixes #18592