Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

Changes:
- SolidColorBrush.GetHashCode(): Add null check for Color
- GradientStop.GetHashCode(): Use HashCode.Combine for proper hashing
- ImageBrush.GetHashCode(): Use HashCode.Combine for proper hashing
- Add XAML unit test for x:Name on resources

Close #28805

Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

The fix uses base.GetHashCode() (identity-based) instead of hashing
the mutable Color/ImageSource properties. This is correct because:
1. These are mutable reference types - their hash shouldn't change
2. The original implementation was incorrect anyway (hash based on
   mutable state breaks dictionary/hashset semantics)

Changes:
- SolidColorBrush.GetHashCode(): Use base.GetHashCode()
- GradientStop.GetHashCode(): Use base.GetHashCode()
- ImageBrush.GetHashCode(): Use base.GetHashCode()
- Add XAML unit test for x:Name on resources

Close #28805

Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

The fix uses base.GetHashCode() (identity-based) instead of hashing
the mutable Color/ImageSource properties. This is correct because:
1. These are mutable reference types - their hash shouldn't change
2. The original implementation was incorrect anyway (hash based on
   mutable state breaks dictionary/hashset semantics)

Changes:
- SolidColorBrush.GetHashCode(): Use base.GetHashCode()
- GradientStop.GetHashCode(): Use base.GetHashCode()
- ImageBrush.GetHashCode(): Use base.GetHashCode()
- Add XAML unit test for x:Name on resources

Close #28805

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## Description

Fixes #28711
Close #28805

When using `x:Name` on a `SolidColorBrush` (or other brush types) in
`ContentPage.Resources`, a `NullReferenceException` was thrown. This
occurred because `GetHashCode()` was called during NameScope
registration before the `Color` property was set.

## Root Cause

When XAML elements with `x:Name` are registered in the NameScope, the
registration calls `GetHashCode()` on the object (for dictionary
storage). The original `SolidColorBrush.GetHashCode()` implementation
directly called `Color.GetHashCode()` without null-checking, causing a
crash when `Color` was still `null`.

## Changes

- **`SolidColorBrush.GetHashCode()`**: Added null-safe operator
(`Color?.GetHashCode() ?? 0`) and use `HashCode.Combine()` for proper
hashing
- **`GradientStop.GetHashCode()`**: Updated to use `HashCode.Combine()`
for proper hashing (fixes same potential issue)
- **`ImageBrush.GetHashCode()`**: Updated to use `HashCode.Combine()`
for proper hashing (fixes same potential issue)

The changes use `HashCode.Combine()` for non-NETSTANDARD targets and a
properly implemented hash formula for NETSTANDARD compatibility.

## Test

Added XAML unit test `Maui28711.xaml`/`.xaml.cs` that:
- Creates a page with `x:Name="namedBrush"` on a `SolidColorBrush` in
Resources
- Verifies the page loads without crashing across all 3 XAML inflators
(Runtime, XamlC, SourceGen)
- Validates the named field exists and the Color is set correctly

## Comparison with PR #28805

This PR supersedes #28805 with the following improvements:
- Test is properly placed in `Xaml.UnitTests` with `XamlInflator`
parameter (as requested by reviewer @StephaneDelcroix)
- Added `unchecked` to the NETSTANDARD path to properly handle potential
overflow
- Cleaner implementation following existing patterns in the codebase

Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

The fix uses base.GetHashCode() (identity-based) instead of hashing
the mutable Color/ImageSource properties. This is correct because:
1. These are mutable reference types - their hash shouldn't change
2. The original implementation was incorrect anyway (hash based on
   mutable state breaks dictionary/hashset semantics)

Changes:
- SolidColorBrush.GetHashCode(): Use base.GetHashCode()
- GradientStop.GetHashCode(): Use base.GetHashCode()
- ImageBrush.GetHashCode(): Use base.GetHashCode()
- Add XAML unit test for x:Name on resources

Close #28805

Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

The fix uses base.GetHashCode() (identity-based) instead of hashing
the mutable Color/ImageSource properties. This is correct because:
1. These are mutable reference types - their hash shouldn't change
2. The original implementation was incorrect anyway (hash based on
   mutable state breaks dictionary/hashset semantics)

Changes:
- SolidColorBrush.GetHashCode(): Use base.GetHashCode()
- GradientStop.GetHashCode(): Use base.GetHashCode()
- ImageBrush.GetHashCode(): Use base.GetHashCode()
- Add XAML unit test for x:Name on resources

Close #28805

Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

The fix uses base.GetHashCode() (identity-based) instead of hashing
the mutable Color/ImageSource properties. This is correct because:
1. These are mutable reference types - their hash shouldn't change
2. The original implementation was incorrect anyway (hash based on
   mutable state breaks dictionary/hashset semantics)

Changes:
- SolidColorBrush.GetHashCode(): Use base.GetHashCode()
- GradientStop.GetHashCode(): Use base.GetHashCode()
- ImageBrush.GetHashCode(): Use base.GetHashCode()
- Add XAML unit test for x:Name on resources

Close #28805

Fixes #28711

When using x:Name on a SolidColorBrush in ContentPage.Resources, a
NullReferenceException was thrown because GetHashCode was called
(during NameScope registration) before Color was set.

The fix uses base.GetHashCode() (identity-based) instead of hashing
the mutable Color/ImageSource properties. This is correct because:
1. These are mutable reference types - their hash shouldn't change
2. The original implementation was incorrect anyway (hash based on
   mutable state breaks dictionary/hashset semantics)

Changes:
- SolidColorBrush.GetHashCode(): Use base.GetHashCode()
- GradientStop.GetHashCode(): Use base.GetHashCode()
- ImageBrush.GetHashCode(): Use base.GetHashCode()
- Add XAML unit test for x:Name on resources

Close #28805