[ObjCRuntime] Fix GC race in RetainAndAutoreleaseHandle causing intermittent SIGSEGV (#24814)

After obj.GetHandle() extracts the native handle, the GC can finalize obj (a NativeObject, not protected by ObjC runtime reference tracking) before DangerousRetain executes. This causes a use-after-free when the finalizer calls CFRelease on the handle before it's been retained.

Add GC.KeepAlive(obj) to ensure the managed wrapper survives until after the retain+autorelease has completed.

This fixes intermittent SIGSEGV crashes in CGBitmapContext.CreateAdaptive tests (and potentially any other caller of RetainAndAutoreleaseHandle with NativeObject-derived types).

---------

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

Author: rolfbjarne

src/ObjCRuntime/Runtime.cs

@@ -2337,9 +2337,9 @@ internal static NativeHandle RetainAndAutoreleaseHandle (INativeObject? obj)
 		{
 			if (obj is null)
 				return NativeHandle.Zero;
-#pragma warning disable RBI0014
-			return RetainAndAutoreleaseHandle (obj.GetHandle ());
-#pragma warning restore RBI0014
+			var rv = RetainAndAutoreleaseHandle (obj.GetHandle ());
+			GC.KeepAlive (obj);
+			return rv;
 		}
 
 		/// <summary>Retain and autorelease the given handle, then return the handle.</summary>

tests/cecil-tests/HandleSafety.KnownFailures.cs

@@ -149,7 +149,6 @@ public partial class HandleSafetyTest {
 			"ObjCRuntime.RegistrarHelper.INativeObject_managed_to_native (System.IntPtr, ObjCRuntime.INativeObject, ObjCRuntime.INativeObject, System.Boolean)",
 			"ObjCRuntime.Runtime.ConvertSmartEnumToNSString (System.IntPtr)",
 			"ObjCRuntime.Runtime.GetHandleForINativeObject (System.IntPtr)",
-			"ObjCRuntime.Runtime.RetainAndAutoreleaseHandle (ObjCRuntime.INativeObject)",
 			"ObjCRuntime.Runtime.RetainAndAutoreleaseNativeObject (ObjCRuntime.INativeObject)",
 			"ObjCRuntime.Runtime.RetainAndAutoreleaseNSObject (Foundation.NSObject)",
 			"ObjCRuntime.Runtime.RetainNativeObject (ObjCRuntime.INativeObject)",