[Foundation] Fix nullability in NSObject. (#24481)

This is file 43 of 47 files with nullability disabled in Foundation.

* Enables nullability.
* Removes SupportedOSPlatform attributes without version numbers from NSObject and NSObservedChange classes.
* Replaces XML include attributes with inlined content from NSObjectFlag.xml and NSObservedChange.xml.
* Removes 'To be added' XML comments with proper documentation.
* Adds missing XML documentation for public and protected members.
* Improves XML comments for language, grammar, consistency and correctness.
* Adds see cref attributes for type and member references.
* Removes unnecessary whitespace in XML comments.

Contributes towards #17285.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: rolfbjarne

docs/api/Foundation/NSObjectFlag.xml

@@ -442,7 +442,7 @@ public void GetComponents (out nfloat [] components)
 		/// <summary>To be added.</summary>
 		///         <returns>To be added.</returns>
 		///         <remarks>To be added.</remarks>
-		public override string ToString ()
+		public override string? ToString ()
 		{
 			try {
 				string name = this.ColorSpaceName;

docs/api/Foundation/NSObservedChange.xml

@@ -110,7 +110,7 @@ public unsafe bool Equals (CBUUID? obj)
 		///         <summary>To be added.</summary>
 		///         <returns>To be added.</returns>
 		///         <remarks>To be added.</remarks>
-		public override bool Equals (object obj)
+		public override bool Equals (object? obj)
 		{
 			return base.Equals (obj);
 		}

src/AppKit/NSColor.cs

@@ -144,7 +144,7 @@ public static CFException FromCFError (IntPtr cfErrorHandle, bool release)
 				using (var userInfo = new NSDictionary (cfUserInfo)) {
 					foreach (var i in userInfo) {
 						if (i.Key is not null)
-							e.Data.Add (i.Key.ToString (), i.Value?.ToString ());
+							e.Data.Add (i.Key.ToString () ?? "", i.Value?.ToString () ?? "");
 					}
 				}
 			}

src/CoreBluetooth/CBUUID.cs

@@ -89,19 +89,19 @@ internal NSDictionary ToDictionary ()
 			// Tracking exists only in iOS6+, before this the field is null (and would throw if used)
 			if (CIDetector.Tracking is not null && TrackingEnabled is not null) {
 				keys.Add (CIDetector.Tracking);
-				values.Add (NSObject.FromObject (TrackingEnabled.Value));
+				values.Add (NSObject.FromObject (TrackingEnabled.Value)!);
 			}
 
 			// EyeBlink exists only in iOS7+, before this the field is null (and would throw if used)
 			if (CIDetector.EyeBlink is not null && EyeBlink is not null) {
 				keys.Add (CIDetector.EyeBlink);
-				values.Add (NSObject.FromObject (EyeBlink.Value));
+				values.Add (NSObject.FromObject (EyeBlink.Value)!);
 			}
 
 			// Smile exists only in iOS7+, before this the field is null (and would throw if used)
 			if (CIDetector.Smile is not null && Smile is not null) {
 				keys.Add (CIDetector.Smile);
-				values.Add (NSObject.FromObject (Smile.Value));
+				values.Add (NSObject.FromObject (Smile.Value)!);
 			}
 			// AspectRation exists only in iOS8+, before this the field is null (and would throw if used)
 			if (CIDetector.AspectRatio is not null && AspectRatio is not null) {

src/CoreFoundation/CFException.cs

@@ -633,7 +633,7 @@ protected void SetArrayValue<T> (NSString key, T []? values)
 			if (NullCheckObjectAndRemoveKey (key, values)) {
 				var nsValues = new NSObject [values.Length];
 				for (var i = 0; i < values.Length; i++)
-					nsValues [i] = NSObject.FromObject (values [i]);
+					nsValues [i] = NSObject.FromObject (values [i])!;
 				Dictionary [key] = NSArray.FromNSObjects (nsValues);
 			}
 		}

src/CoreImage/CIDetectorOptions.cs

@@ -64,9 +64,9 @@ public NSActionDispatcher (Action action)
 	[Register ("__MonoMac_NSSynchronizationContextDispatcher")]
 	internal sealed class NSSynchronizationContextDispatcher : NSDispatcher {
 		readonly SendOrPostCallback d;
-		readonly object state;
+		readonly object? state;
 
-		public NSSynchronizationContextDispatcher (SendOrPostCallback d, object state)
+		public NSSynchronizationContextDispatcher (SendOrPostCallback d, object? state)
 		{
 			if (d is null)
 				throw new ArgumentNullException (nameof (d));
@@ -149,7 +149,7 @@ internal sealed class NSAsyncSynchronizationContextDispatcher : NSAsyncDispatche
 		SendOrPostCallback? d;
 		object? state;
 
-		public NSAsyncSynchronizationContextDispatcher (SendOrPostCallback d, object state)
+		public NSAsyncSynchronizationContextDispatcher (SendOrPostCallback d, object? state)
 		{
 			if (d is null)
 				throw new ArgumentNullException (nameof (d));