[xabt] Use Load() in RunPipeline instead of pre-loading all assemblies

The pre-loading approach in #11208 eagerly loads ALL ResolvedAssemblies
into the resolver cache. If any of those assemblies reference
netstandard.dll (e.g. netstandard2.1 NuGet packages), Cecil lazy
reference resolution will fail with FileNotFoundException since
netstandard.dll is not in the search directories.

Instead, use resolver.Load(source.ItemSpec) in RunPipeline to load each
assembly from its exact path when processed. This ensures the correct
TFM version is loaded without eagerly loading all assemblies upfront.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tasks/AssemblyModifierPipeline.cs

@@ -102,19 +102,13 @@ public override bool RunTask ()
 
 				var resolver = new DirectoryAssemblyResolver (this.CreateTaskLogger (), loadDebugSymbols: ReadSymbols, loadReaderParameters: readerParameters);
 
-				// Add SearchDirectories and pre-load ResolvedAssemblies into the resolver cache.
-				// Pre-loading ensures the correct TFM version is cached before any Cecil lazy
-				// reference resolution can find wrong-TFM copies from search directories (e.g.,
-				// a net11.0 copy in a referencing project's output directory).
+				// Add SearchDirectories for the current architecture's ResolvedAssemblies
 				foreach (var kvp in perArchAssemblies [sourceArch]) {
 					ITaskItem assembly = kvp.Value;
 					var path = Path.GetFullPath (Path.GetDirectoryName (assembly.ItemSpec));
 					if (!resolver.SearchDirectories.Contains (path)) {
 						resolver.SearchDirectories.Add (path);
 					}
-					if (resolver.Load (assembly.ItemSpec) == null) {
-						Log.LogDebugMessage ($"Could not pre-load assembly '{assembly.ItemSpec}' into resolver cache.");
-					}
 				}
 
 				// Set up the FixAbstractMethodsStep and AddKeepAlivesStep
@@ -166,7 +160,14 @@ protected virtual void BuildPipeline (AssemblyPipeline pipeline, MSBuildLinkCont
 
 	void RunPipeline (AssemblyPipeline pipeline, ITaskItem source, ITaskItem destination)
 	{
-		var assembly = pipeline.Resolver.GetAssembly (source.ItemSpec);
+		// Use Load with the exact ItemSpec path to ensure the correct TFM version
+		// is loaded, rather than GetAssembly which strips the path and resolves by
+		// name through search directories (which may find wrong-TFM copies).
+		var assembly = pipeline.Resolver.Load (source.ItemSpec);
+		if (assembly == null) {
+			Log.LogDebugMessage ($"Could not load assembly '{source.ItemSpec}', skipping.");
+			return;
+		}
 
 		var context = new StepContext (source, destination) {
 			CodeGenerationTarget = codeGenerationTarget,