Fixed CI failure : Restore BackButtonBehavior IsEnabled after CanExecute changes (#34668)

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### Issue details
This PR #28734 causes the unit test below

1. BackButtonUpdatesWhenSetToNewCommand
2. BackButtonDisabledWhenCommandDisabled

### Description of Change

The root cause is [IsEnabledCore { set => SetValue(IsEnabledProperty,
value && IsEnabled);
}](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
— a circular read: once
[IsEnabledProperty](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
is false, reading
[IsEnabled](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
always returns false, so the property is permanently stuck.

The fix introduces two backing fields to track the concerns
independently:

-
[_userDefinedIsEnabled](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
— written only by the developer via
[IsEnabled](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
-
[_commandEnabled](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
— written only by
[CanExecute](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
via
[IsEnabledCore](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
-
[IsEnabledProperty](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
is always [_userDefinedIsEnabled &&
_commandEnabled](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html),
eliminating the circular read.

- BackButtonDisabledWhenCommandDisabled — When
[CanExecute](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
flips to true and
[ChangeCanExecute()](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
is called,
[_commandEnabled](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
updates correctly and the back button re-enables.

- BackButtonUpdatesWhenSetToNewCommand — Setting [Command =
null](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
resets [_commandEnabled =
true](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html),
so the back button re-enables as expected.


### Test fixed
Unit tests:
BackButtonUpdatesWhenSetToNewCommand
BackButtonDisabledWhenCommandDisabled

Author: Shalini-Ashokan

src/Controls/src/Core/Shell/BackButtonBehavior.cs

@@ -62,13 +62,22 @@ public ImageSource IconOverride
 			set { SetValue(IconOverrideProperty, value); }
 		}
 
+		// Tracks the value explicitly set by the user (default matches IsEnabledProperty default of true).
+		bool _userDefinedIsEnabled = true;
+		// Tracks the enabled state derived from the command's CanExecute result.
+		bool _commandEnabled = true;
+
 		/// <summary>
 		/// Gets or sets a value indicating whether the back button is enabled. This is a bindable property.
 		/// </summary>
 		public bool IsEnabled
 		{
 			get { return (bool)GetValue(IsEnabledProperty); }
-			set { SetValue(IsEnabledProperty, value); }
+			set
+			{
+				_userDefinedIsEnabled = value;
+				SetValue(IsEnabledProperty, _userDefinedIsEnabled && _commandEnabled);
+			}
 		}
 
 		/// <summary>
@@ -89,7 +98,14 @@ public string TextOverride
 			set { SetValue(TextOverrideProperty, value); }
 		}
 
-		bool IsEnabledCore { set => SetValue(IsEnabledProperty, value && IsEnabled); }
+		bool IsEnabledCore
+		{
+			set
+			{
+				_commandEnabled = value;
+				SetValue(IsEnabledProperty, _userDefinedIsEnabled && value);
+			}
+		}
 
 		static void OnCommandChanged(BindableObject bindable, object oldValue, object newValue)
 		{