Add XAML unit testing guidelines

.github/instructions/xaml-unittests.instructions.md

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+---
+description: "Guidelines for writing XAML unit tests in the Controls.Xaml.UnitTests project"
+---
+
+# XAML Unit Testing Guidelines
+
+This guide provides conventions for writing XAML unit tests in `src/Controls/tests/Xaml.UnitTests/`.
+
+## 1. File Naming and Location for Issue Tests
+
+When writing a test for a GitHub issue:
+
+- **Location**: Place the test file in the `Issues/` folder
+- **Naming convention**: `MauiXXXXX.xaml` and `MauiXXXXX.xaml.cs` (where XXXXX is the GitHub issue number)
+
+**Example:**
+```
+src/Controls/tests/Xaml.UnitTests/Issues/
+├── Maui12345.xaml
+└── Maui12345.xaml.cs
+```
+
+## 2. Basic Test Pattern
+
+Most unit tests will:
+1. Check that the XAML page can be created/inflated
+2. Perform some assertions on the created page
+3. Apply to all `[XamlInflator]` test variations (runtime, XamlC, SourceGen)
+
+**XAML file (Maui12345.xaml):**
+```xaml
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Maui12345">
+    <!-- Your test content -->
+</ContentPage>
+```
+
+**Code-behind (Maui12345.xaml.cs):**
+```csharp
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public partial class Maui12345 : ContentPage
+{
+    public Maui12345()
+    {
+        InitializeComponent();
+    }
+
+    [TestFixture]
+    class Tests
+    {
+        [Test]
+        public void MyTest([Values] XamlInflator inflator)
+        {
+            var page = new Maui12345(inflator);
+            Assert.That(page, Is.Not.Null);
+            // Additional assertions
+        }
+    }
+}
+```
+
+**Note**: Only add `[SetUp]` and `[TearDown]` methods if your test requires special initialization (e.g., setting up `DispatcherProvider` or `Application.Current`). Most tests don't need them.
+
+## 3. Testing IL Correctness (XamlC) or Source Generation
+
+When the test specifically validates compile-time behavior for a specific inflator:
+
+**XamlC (IL generation):**
+```csharp
+[Test]
+public void MyTest([Values] XamlInflator inflator)
+{
+    if (inflator == XamlInflator.XamlC)
+        Assert.Throws<BuildException>(() => MockCompiler.Compile(typeof(Maui12345)));
+    else
+    {
+        var page = new Maui12345(inflator);
+        Assert.That(page, Is.Not.Null);
+    }
+}
+```
+
+**Source Generation:**
+```csharp
+using static Microsoft.Maui.Controls.Xaml.UnitTests.MockSourceGenerator;
+
+[Test]
+public void MyTest([Values] XamlInflator inflator)
+{
+    if (inflator == XamlInflator.SourceGen)
+    {
+        var result = CreateMauiCompilation()
+            .WithAdditionalSource(
+"""
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+[XamlProcessing(XamlInflator.Runtime, true)]
+public partial class Maui12345 : ContentPage
+{
+    public Maui12345() => InitializeComponent();
+}
+""")
+            .RunMauiSourceGenerator(typeof(Maui12345));
+        Assert.That(result.Diagnostics, Is.Empty); // or Is.Not.Empty for error cases
+    }
+    else
+    {
+        var page = new Maui12345(inflator);
+        Assert.That(page, Is.Not.Null);
+    }
+}
+```
+
+## 4. Handling Invalid Code Generation
+
+When testing scenarios where code generation is **expected to fail** or produce invalid code, use special file extensions to prevent build failures:
+
+| Extension | Behavior | Use Case |
+|-----------|----------|----------|
+| `.rt.xaml` | Runtime inflation only | XamlC/SourceGen should fail, but runtime should work |
+| `.rtsg.xaml` | Runtime + SourceGen only | XamlC should fail |
+| `.rtxc.xaml` | Runtime + XamlC only | SourceGen should fail |
+
+**Why?** These extensions prevent the compiler and build tasks from processing the XAML file, allowing your test to execute and verify the expected behavior.
+
+**Example:**
+```
+Issues/
+├── Maui12345.rt.xaml      # Only inflated at runtime (compiler/sourcegen skip)
+└── Maui12345.xaml.cs
+```
+
+## Quick Reference
+
+| Scenario | Location | Naming | Special Extension |
+|----------|----------|--------|-------------------|
+| Issue test | `Issues/` | `MauiXXXXX.*` | None (unless invalid codegen) |
+| Invalid XamlC | `Issues/` | `MauiXXXXX.rt.xaml` | `.rt.xaml` |
+| Invalid SourceGen | `Issues/` | `MauiXXXXX.rt.xaml` or `.rtxc.xaml` | Depends on what should fail |
+| Test XamlC IL | Anywhere | Any | Use `MockCompiler` |
+| Test SourceGen | Anywhere | Any | Use `MockSourceGenerator` |
+
+## Related Documentation
+
+- `.github/copilot-instructions.md` - General MAUI development guidelines
+- `.github/instructions/uitests.instructions.md` - UI testing guidelines (different from XAML unit tests)