Guard KVC access for accessibilityCustomContent with responds(to:) (#314)

Prevent potential NSUnknownKeyException crash when value(forKey:) is
called on NSObject subclasses that don't have the
accessibilityCustomContent key and override valueForUndefinedKey: to
throw.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: RoyalPineapple

Sources/AccessibilitySnapshot/Parser/Swift/Classes/AccessibilityHierarchyParser.swift

@@ -852,7 +852,9 @@ private extension NSObject {
 
             // SwiftUI creates internal accessibility proxy nodes that don't explicitly conform to AXCustomContentProvider
             // but do expose accessibilityCustomContent via KVC
-            if let content = value(forKey: "accessibilityCustomContent") as? [AXCustomContent] {
+            if responds(to: Selector(("accessibilityCustomContent"))),
+               let content = value(forKey: "accessibilityCustomContent") as? [AXCustomContent]
+            {
                 return content.map { .init(from: $0) }
             }
         }