[runtime] Call mono_debugger_agent_unhandled_exception when an except…

…ion is marshalled.

This means the debugger will be notified of all marshalled exceptions, and
consider them unhandled (and break the debugged app into the debugger).

There's no corresponding change for CoreCLR, because we have no configuration
where we can attach a debugger to a CoreCLR-based app (VSCode doesn't support
macOS, and NativeAOT doesn't support debugging).

Partial fix for #15037.
Fixes dotnet/maui#7176.
Ref: dotnet/android#6106

runtime/exports.t4

@@ -839,6 +839,12 @@
 			XamarinRuntime = RuntimeMode.MonoVM,
 		},
 
+		new Export ("void", "mono_debugger_agent_unhandled_exception",
+			"MonoException *", "e"
+		) {
+			XamarinRuntime = RuntimeMode.MonoVM,
+		},
+
 		#endregion
 
 		#region mini/mono-private-unstable.

runtime/runtime.m

@@ -2348,6 +2348,11 @@ -(void) xamarinSetFlags: (enum XamarinGCHandleFlags) flags;
 	if (exception == NULL)
 		return;
 
+#if !defined (CORECLR_RUNTIME)
+	if (mono_is_debugger_attached ())
+		mono_debugger_agent_unhandled_exception ((MonoException *) exception);
+#endif
+
 	MarshalManagedExceptionMode mode;
 	GCHandle exception_gchandle = INVALID_GCHANDLE;