Guard Mono-specific AOT targets for CoreCLR runtime, add XA1042 warning

[Copilot]

Setting <RunAOTCompilation>true</RunAOTCompilation> in a project file (e.g. Directory.Build.props) causes build failures when switching from Mono to CoreCLR, because MonoAOTCompiler runs unconditionally regardless of the active runtime.

Changes

• Microsoft.Android.Sdk.Aot.targets — Add '$(_AndroidRuntime)' == 'MonoVM' guard to the ImportGroup (MonoAOTCompiler SDK imports) and _AndroidAot target condition
• Microsoft.Android.Sdk.DefaultProperties.targets — After RunAOTCompilation defaults are resolved, detect Mono-specific properties (RunAOTCompilation, EnableLLVM) set with non-MonoVM runtimes, reset them to false, and set flags for warning emission
• Xamarin.Android.Common.targets — Emit new XA1042 warning when the flags are set; add $(PublishAot) condition to _NdkRequired so NativeAOT always requires the Android NDK
• Resources.resx / Resources.Designer.cs — New XA1042 message
• xa1042.md — Documentation for the new warning
• AotTests.cs — Two tests: RunAOTCompilationWithCoreClrWarnsAndSkipsMonoAot and EnableLLVMWithCoreClrWarnsAndIsIgnored
• AndroidDependenciesTests.cs — New NativeAotRequiresNdk test validating that NativeAOT always lists the NDK as a dependency

Example warning

warning XA1042: The MSBuild property 'RunAOTCompilation' is not compatible with the CoreCLR runtime and will be ignored.
Either remove the property or guard it with a condition: Condition="'$(UseMonoRuntime)' == 'true'"

Migration guidance

<!-- Before: breaks when switching to CoreCLR -->
<RunAOTCompilation>true</RunAOTCompilation>

<!-- After: only applies to Mono -->
<PropertyGroup Condition=" '$(UseMonoRuntime)' == 'true' ">
  <RunAOTCompilation>true</RunAOTCompilation>
</PropertyGroup>

[jonathanpeppers]

@copilot we need to fix this test:

GetDependencyNdkRequiredConditions("EnableLLVM",True,NativeAOT)

When $(PublishAot) is true, we should require the Android NDK.

We may want to add an additional test so this is clearer.

[Copilot]

Pull request overview

This PR prevents Mono AOT build logic from running when a project is configured for non-Mono runtimes (CoreCLR/NativeAOT), and adds a dedicated warning (XA1042) to guide users who have Mono-only AOT properties set unconditionally (e.g., via Directory.Build.props).

Changes:

• Guard MonoAOTCompiler SDK imports and _AndroidAot execution so they only run for MonoVM.
• Detect RunAOTCompilation / EnableLLVM set under non-Mono runtimes, reset them to false, and emit XA1042 with actionable guidance.
• Add XA1042 resource + documentation and add tests validating warning emission and successful builds under CoreCLR.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.Aot.targets	Restricts MonoAOTCompiler SDK imports and _AndroidAot target to MonoVM runtime only.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets	Detects Mono-only properties under non-Mono runtimes, sets XA1042 flags, and resets the properties to false.
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets	Emits XA1042 warnings when the corresponding flags are set.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Adds localized XA1042 warning message template.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Regenerates strongly-typed resource accessor for XA1042.
Documentation/docs-mobile/messages/xa1042.md	Adds end-user documentation and migration guidance for XA1042.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/AotTests.cs	Adds tests ensuring CoreCLR builds succeed and XA1042 is emitted when Mono-only properties are set.

Files not reviewed (1)

• src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs: Language not supported

[Copilot]

@copilot we need to fix this test:

GetDependencyNdkRequiredConditions("EnableLLVM",True,NativeAOT)

...

Fixed in b2456ac. The issue was that my XA1042 change resets EnableLLVM to false for non-MonoVM runtimes, but NativeAOT still needs the NDK (it uses the NDK's clang compiler).

Two changes:

1. Added <_NdkRequired Condition="'$(PublishAot)' == 'true'">true</_NdkRequired> to the GetAndroidDependencies target so NativeAOT always requires NDK regardless of EnableLLVM.
2. Added a dedicated NativeAotRequiresNdk test that validates NativeAOT requires NDK without relying on any Mono-specific properties.