[Android] Gradient background with ImageButton

[kubaflo]

Issues Fixed

Fixes #23119

<ImageButton
        AutomationId="imageButton"
        WidthRequest="100"
        HeightRequest="100"
        HorizontalOptions="Center">
        <ImageButton.Background>
                <RadialGradientBrush
                        Center="0.5,0.5"
                        Radius="0.5">
                        <GradientStop
                                Color="green"
                                Offset="0.0"/>
                        <GradientStop
                                Color="red"
                                Offset="1.0"/>
                </RadialGradientBrush>
        </ImageButton.Background>
</ImageButton>

Before	After

[dotnet-policy-service]

Hey there @kubaflo! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jsuarezruiz]

/rebase

[jsuarezruiz]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[jfversluis]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

Image was updated

[jfversluis]

Still see a relevant test failure

VisualTestUtils.VisualTestFailedException :
Snapshot different than baseline: GradientBackgroundShouldWorkWithImageButton.png (size differs - baseline is 1920x1080 pixels, actual is 789x563 pixels)
If the correct baseline has changed (this isn't a a bug), then update the baseline image.
See test attachment or download the build artifacts to get the new snapshot file.

[Copilot]

Pull Request Overview

This PR fixes an issue where ImageButton controls on Android were not properly displaying gradient backgrounds, showing solid colors instead of the expected gradients. The fix involves updating the background when the ImageButton's layout changes.

• Adds a layout change event handler to the Android ImageButton handler to trigger background updates
• Includes comprehensive UI tests to verify gradient background functionality
• Tests both RadialGradientBrush and LinearGradientBrush with ImageButton and Button controls

Reviewed Changes

Copilot reviewed 4 out of 8 changed files in this pull request and generated no comments.

File	Description
src/Core/src/Handlers/ImageButton/ImageButtonHandler.Android.cs	Adds layout change event handling to properly update gradient backgrounds on Android
src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue23119.cs	Implements automated UI test to verify gradient background rendering
src/Controls/tests/TestCases.HostApp/Issues/Issue23119.xaml.cs	Creates test page code-behind for the gradient background issue reproduction
src/Controls/tests/TestCases.HostApp/Issues/Issue23119.xaml	Defines XAML test page with various gradient background scenarios for validation

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