[TrimmableTypeMap] Emit XA4212 for custom IJavaObject types on the trimmable path

[simonrozsival]

Root-cause fix (replaces the earlier test skip). The XA4212 "custom IJavaObject not supported" diagnostic was produced only by the Cecil-based XAJavaTypeScanner, which runs on the managed path (AssemblyModifierPipeline in _RunAfterILLinkAdditionalSteps) and the llvm-ir path. On the trimmable typemap path — now the NativeAOT default — that post-ILLink pipeline is intentionally skipped as dead work, so a managed class implementing Android.Runtime.IJavaObject without deriving from Java.Lang.Object/Throwable was silently ignored: no diagnostic, and the type is absent from the typemap, failing at runtime.

Detect this case directly in JavaPeerScanner, which already scans every type: a class that is not a Java peer, is not a System.Exception subclass, and implements Android.Runtime.IJavaObject (directly, transitively via an interface, or through a base class) now emits XA4212. Thread AndroidErrorOnCustomJavaObject through the GenerateTrimmableTypeMap task and generator so it errors by default and warns when disabled, matching the legacy behavior and message format. Un-skip the XA4212 test.

Verified locally: XA4212(NativeAOT) and XA4212(CoreCLR) pass, 614 trimmable-typemap generator unit tests pass, and BuildBasicApplicationAppCompat (NativeAOT + CoreCLR) builds clean with no false-positive XA4212.

[Copilot]

Pull request overview

This PR restores the XA4212 diagnostic (“custom IJavaObject not supported”) on the trimmable typemap path (now the NativeAOT default) by detecting offending managed types during the Reflection.Metadata-based scan, and threads the existing $(AndroidErrorOnCustomJavaObject) behavior through to the trimmable pipeline.

Changes:

• Add custom-IJavaObject detection to JavaPeerScanner and emit XA4212 as error vs warning based on a new errorOnCustomJavaObject knob.
• Thread AndroidErrorOnCustomJavaObject into GenerateTrimmableTypeMap and into the trimmable typemap MSBuild target invocation.
• Add XA4212 to Resources.resx/Designer and extend the trimmable typemap logger interface + test logger.

Show a summary per file

File	Description
tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TrimmableTypeMapGeneratorTests.cs	Extends test logger with XA4212 logging hooks for generator-level tests.
src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs	Threads ErrorOnCustomJavaObject into the trimmable generator and logs XA4212.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Adds localized XA4212 message.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Adds the strongly-typed XA4212 resource accessor.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.targets	Passes $(AndroidErrorOnCustomJavaObject) into GenerateTrimmableTypeMap.
src/Microsoft.Android.Sdk.TrimmableTypeMap/TrimmableTypeMapGenerator.cs	Adds errorOnCustomJavaObject plumbing into scanning.
src/Microsoft.Android.Sdk.TrimmableTypeMap/Scanner/JavaPeerScanner.cs	Implements custom-IJavaObject detection and XA4212 emission in the scanner.
src/Microsoft.Android.Sdk.TrimmableTypeMap/ITrimmableTypeMapLogger.cs	Adds XA4212 error/warning logging methods to the generator logger contract.

Copilot's findings

Files not reviewed (1)

• src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs: Generated file

• Files reviewed: 7/8 changed files
• Comments generated: 4

[simonrozsival]

@copilot resolve the merge conflicts in this pull request and implement code review suggestions

[Copilot]

@copilot resolve the merge conflicts in this pull request and implement code review suggestions

Done in ad7d62ab9. I resolved the merge conflicts with main and implemented the requested review suggestions (default errorOnCustomJavaObject alignment, IsCustomJavaObject check ordering, and distinct test logging for XA4212 errors vs warnings).