[TrimmableTypeMap] Always wire registerJniNativesFn, fail loudly if called

Always set the registerJniNativesFn function pointer during runtime
initialization, even in trimmable mode. This ensures that if a JCW
incorrectly calls Runtime.register(typeName, klass, methods) instead
of Runtime.registerNatives(klass), the call flows through to C# where
TrimmableTypeMapTypeManager.RegisterNativeMembers throws
UnreachableException with a clear diagnostic message.

Previously, the function pointer was set to nullptr for trimmable mode,
causing the C++ side to silently drop the call — making JCW generation
bugs invisible at runtime.

Also adds a unit test verifying that trimmable JCWs never emit
Runtime.register().

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

Author: simonrozsival

src/Mono.Android/Android.Runtime/JNIEnvInit.cs

@@ -178,9 +178,7 @@ internal static unsafe void Initialize (JnienvInitializeArgs* args)
 
 			args->propagateUncaughtExceptionFn = (IntPtr)(delegate* unmanaged<IntPtr, IntPtr, IntPtr, void>)&PropagateUncaughtException;
 
-			if (!RuntimeFeature.TrimmableTypeMap) {
-				args->registerJniNativesFn = (IntPtr)(delegate* unmanaged<IntPtr, int, IntPtr, IntPtr, int, void>)&RegisterJniNatives;
-			}
+			args->registerJniNativesFn = (IntPtr)(delegate* unmanaged<IntPtr, int, IntPtr, IntPtr, int, void>)&RegisterJniNatives;
 			RunStartupHooksIfNeeded ();
 			SetSynchronizationContext ();
 		}

src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMapTypeManager.cs

@@ -67,7 +67,7 @@ public override void RegisterNativeMembers (
 			ReadOnlySpan<char> methods)
 	{
 		throw new UnreachableException (
-			$"RegisterNativeMembers should not be called in the trimmable typemap path. " +
-			$"Native methods for '{type.FullName}' should be registered by JCW static initializer blocks.");
+			$"Trimmable JCWs should only call Runtime.registerNatives(klass), never Runtime.register(typeName, klass, methods). " +
+			$"Type '{type.FullName}' triggered a Runtime.register() call, which indicates a JCW generation bug.");
 	}
 }

tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/JcwJavaSourceGeneratorTests.cs

@@ -134,6 +134,15 @@ public void Generate_AcwType_HasRegisterNativesStaticBlock ()
 			AssertContainsLine ("mono.android.Runtime.registerNatives (MainActivity.class);\n", java);
 		}
 
+		[Fact]
+		public void Generate_AcwType_NeverCallsRuntimeRegister ()
+		{
+			var java = GenerateFixture ("my/app/MainActivity");
+			// Runtime.register("typeName", klass, "methods") must never appear in trimmable JCWs.
+			// Only Runtime.registerNatives(klass) is allowed.
+			Assert.DoesNotContain ("Runtime.register (\"", java);
+		}
+
 		[Fact]
 		public void Generate_ApplicationType_SkipsRegisterNatives ()
 		{