[Shell] Fix InvalidCastException when using QueryPropertyAttribute with nullable types (#33429)

> [!NOTE]
> Are you waiting for the changes in this PR to be merged?
> It would be very helpful if you could [test the resulting
artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from
this PR and let us know in a comment if this change resolves your issue.
Thank you!

## Description

Fixes #33420

This PR fixes a that occurs when using with nullable types during Shell
navigation.

## Root Cause

In `ShellContent.ApplyQueryAttributes`, the code used
`Convert.ChangeType(value, prop.PropertyType)` which fails when the
property type is nullable (e.g., `long?`, `int?`) because
`Convert.ChangeType` does not handle `Nullable<T>` types directly.

When a user navigates with a nullable parameter like:
```csharp
Dictionary<string, object> param = new() {
    { nameof(DetailsPage.ID), (long?)1 }
};
await Shell.Current.GoToAsync(nameof(DetailsPage), param);
```

And the destination page has:
```csharp
[QueryProperty(nameof(ID), nameof(ID))]
public partial class DetailsPage : ContentPage
{
    public long? ID { get; set; }
}
```

The app would crash with `InvalidCastException`.

## Solution

Use `Nullable.GetUnderlyingType()` to extract the underlying type before
conversion:

1. Check if the property type is nullable using
`Nullable.GetUnderlyingType()`
2. If nullable, extract the underlying type (e.g., `long` from `long?`)
3. Convert the value to the underlying type
4. Assign (automatic wrapping in nullable occurs)
5. Handle null values explicitly

## Changes

**Fix:**
- `src/Controls/src/Core/Shell/ShellContent.cs` - Added nullable type
handling in `ApplyQueryAttributes`

**Tests:**
- Added UI test `Issue33420` that reproduces the issue and verifies the
fix
- Test navigates with a nullable `long?` parameter and verifies
navigation succeeds

## Testing

✅ Tests fail without the fix (reproduces the crash)
✅ Tests pass with the fix (navigation works)
✅ Verified on iOS simulator

## Platforms Affected

All platforms (iOS, Android, Windows, MacCatalyst) - Shell navigation
code is shared.

---------

Author: kubaflo

src/Controls/src/Core/Shell/ShellContent.cs

@@ -416,8 +416,18 @@ static void ApplyQueryAttributes(object content, ShellRouteParameters query, She
 							}
 							else
 							{
-								var castValue = Convert.ChangeType(value, prop.PropertyType);
-								prop.SetValue(content, castValue);
+								// Handle nullable types
+								Type targetType = Nullable.GetUnderlyingType(prop.PropertyType) ?? prop.PropertyType;
+								
+								if (value == null)
+								{
+									prop.SetValue(content, null);
+								}
+								else
+								{
+									var castValue = Convert.ChangeType(value, targetType);
+									prop.SetValue(content, castValue);
+								}
 							}
 						}
 					}

src/Controls/tests/TestCases.HostApp/Issues/Issue33420.xaml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<Shell xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+       xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+       xmlns:local="clr-namespace:Maui.Controls.Sample.Issues"
+       x:Class="Maui.Controls.Sample.Issues.Issue33420"
+       Shell.NavBarIsVisible="False">
+    
+    <ShellContent Route="MainPage" ContentTemplate="{DataTemplate local:Issue33420MainPage}" />
+    
+</Shell>

src/Controls/tests/TestCases.HostApp/Issues/Issue33420.xaml.cs

@@ -0,0 +1,95 @@
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 33420, "System.InvalidCastException when using QueryPropertyAttribute with nullable types", PlatformAffected.All)]
+	public partial class Issue33420 : Shell
+	{
+		public Issue33420()
+		{
+			InitializeComponent();
+
+			Routing.RegisterRoute(nameof(Issue33420DetailsPage), typeof(Issue33420DetailsPage));
+		}
+	}
+
+	public class Issue33420MainPage : ContentPage
+	{
+		public Issue33420MainPage()
+		{
+			var button = new Button
+			{
+				Text = "Navigate with Nullable",
+				AutomationId = "NavigateButton"
+			};
+			
+			button.Clicked += async (s, e) =>
+			{
+				try
+				{
+					var param = new Dictionary<string, object>
+					{
+						{ nameof(Issue33420DetailsPage.ID), (long?)1 }
+					};
+					await Shell.Current.GoToAsync(nameof(Issue33420DetailsPage), param);
+				}
+				catch (Exception ex)
+				{
+					// If navigation fails, update button text to indicate error
+					button.Text = $"Error: {ex.GetType().Name}";
+				}
+			};
+
+			Content = new VerticalStackLayout
+			{
+				Children =
+				{
+					new Label
+					{
+						Text = "Test nullable QueryProperty navigation",
+						AutomationId = "MainLabel"
+					},
+					button
+				},
+				Padding = 10,
+				Spacing = 10
+			};
+		}
+	}
+
+	[QueryProperty(nameof(ID), nameof(ID))]
+	public class Issue33420DetailsPage : ContentPage
+	{
+		private readonly Label _resultLabel;
+
+		public long? ID
+		{
+			get => (long?)GetValue(IDProperty);
+			set => SetValue(IDProperty, value);
+		}
+
+		public static readonly BindableProperty IDProperty =
+			BindableProperty.Create(nameof(ID), typeof(long?), typeof(Issue33420DetailsPage), null, propertyChanged: OnIDChanged);
+
+		private static void OnIDChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			if (bindable is Issue33420DetailsPage page)
+			{
+				page._resultLabel.Text = $"Success: ID={newValue}";
+			}
+		}
+
+		public Issue33420DetailsPage()
+		{
+			_resultLabel = new Label
+			{
+				Text = "Waiting for navigation...",
+				AutomationId = "ResultLabel"
+			};
+
+			Content = new VerticalStackLayout
+			{
+				Children = { _resultLabel },
+				Padding = 10
+			};
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue33420.cs

@@ -0,0 +1,31 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue33420 : _IssuesUITest
+{
+	public override string Issue => "System.InvalidCastException when using QueryPropertyAttribute with nullable types";
+
+	public Issue33420(TestDevice device) : base(device) { }
+
+	[Test]
+	[Category(UITestCategories.Shell)]
+	public void NullableQueryPropertyNavigationShouldNotCrash()
+	{
+		// Wait for main page to load
+		App.WaitForElement("MainLabel");
+		
+		// Tap navigation button with nullable parameter
+		App.Tap("NavigateButton");
+
+		// If the bug is present, the app will crash
+		// If fixed, we should navigate successfully to the details page
+		App.WaitForElement("ResultLabel");
+		
+		// Verify the nullable value was passed correctly
+		var resultText = App.FindElement("ResultLabel").GetText();
+		Assert.That(resultText, Is.EqualTo("Success: ID=1"));
+	}
+}