Add unit tests for PrepareAbiItems MSBuild task

[Copilot]

PrepareAbiItems has no test coverage for its mode-to-basename mapping and ABI-specific path construction logic.

Adds PrepareAbiItemsTests.cs with 18 tests covering:

• All 7 valid modes via [TestCase] parameterization (typemap→typemaps, environment→environment, compressed→compressed_assemblies, jniremap→jni_remap, marshal_methods→marshal_methods, runtime_linking→pinvoke_preserve, jni_init→jni_init_funcs)
• Case-insensitive mode matching (TypeMap, TYPEMAP, COMPRESSED, etc.)
• Invalid mode — verifies error is logged and Execute() returns false
• Multiple ABIs — verifies correct item count, paths, and abi metadata for all 4 ABIs
• Single ABI — basic smoke test

[TestCase ("typemap", "typemaps")]
[TestCase ("environment", "environment")]
// ...
public void ValidMode_ProducesCorrectBaseName (string mode, string expectedBase)
{
    var task = new PrepareAbiItems {
        BuildEngine = new MockBuildEngine (TestContext.Out),
        BuildTargetAbis = new [] { "arm64-v8a" },
        NativeSourcesDir = "/native/src",
        Mode = mode,
        Debug = false,
    };
    Assert.IsTrue (task.Execute ());
    Assert.AreEqual ($"/native/src/{expectedBase}.arm64-v8a.ll", task.AssemblySources [0].ItemSpec);
}

[Copilot]

Pull request overview

Adds NUnit coverage for the PrepareAbiItems MSBuild task’s mode→basename mapping, case-insensitive mode handling, and ABI-specific output item construction. This helps prevent regressions in how the task computes .ll source paths and abi metadata.

Changes:

• Introduces a new PrepareAbiItemsTests test fixture with parameterized cases for all supported Mode values.
• Adds tests for case-insensitive Mode matching and invalid mode error behavior.
• Adds tests validating output item count, ItemSpec path construction, and abi metadata across single/multiple ABIs.

Comments suppressed due to low confidence (3)

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/PrepareAbiItemsTests.cs:56

• Same issue here: the expected ItemSpec is built with a hard-coded '/'. Since the task uses Path.Combine, construct the expected path with Path.Combine and use a case-insensitive comparison appropriate for paths.

			Assert.IsTrue (task.Execute ());
			Assert.IsNotNull (task.AssemblySources);
			Assert.AreEqual (1, task.AssemblySources.Length);
			Assert.AreEqual ($"/native/src/{expectedBase}.x86_64.ll", task.AssemblySources [0].ItemSpec);
		}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/PrepareAbiItemsTests.cs:89

• This test also assumes '/' separators in the produced ItemSpec. Use Path.Combine (NativeSourcesDir, $"typemaps.{abis[i]}.ll") (or similar) for expected values so the test passes cross-platform.

			Assert.IsNotNull (task.AssemblySources);
			Assert.AreEqual (abis.Length, task.AssemblySources.Length);
			for (int i = 0; i < abis.Length; i++) {
				Assert.AreEqual ($"/native/src/typemaps.{abis [i]}.ll", task.AssemblySources [i].ItemSpec);
				Assert.AreEqual (abis [i], task.AssemblySources [i].GetMetadata ("abi"));

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/PrepareAbiItemsTests.cs:107

• Hard-coding the expected ItemSpec with '/' will be platform-dependent because PrepareAbiItems uses Path.Combine. Build the expected path with Path.Combine and compare using the same path comparison pattern used in other tests (e.g., StringAssert.AreEqualIgnoringCase).

			Assert.IsNotNull (task.AssemblySources);
			Assert.AreEqual (1, task.AssemblySources.Length);
			Assert.AreEqual ("/output/environment.x86.ll", task.AssemblySources [0].ItemSpec);
			Assert.AreEqual ("x86", task.AssemblySources [0].GetMetadata ("abi"));