Use ASSIGNCOPY for NSAccessibilityElement string setters

[Copilot]

Addresses feedback from PR #362 to use ASSIGNCOPY instead of ASSIGN for string attribute setters in NSAccessibilityElement.m to prevent issues with mutable strings being modified after assignment.

Changes

• String setters: Updated setAccessibilityLabel:, setAccessibilityIdentifier:, setAccessibilityRole:, and setAccessibilitySubrole: to use ASSIGNCOPY
• Initializer: Updated initWithRole:frame:label:parent: to use ASSIGNCOPY for role and label parameters

// Before
- (void) setAccessibilityLabel: (NSString *)label
{
  ASSIGN(_accessibilityLabel, label);  // retains
}

// After
- (void) setAccessibilityLabel: (NSString *)label
{
  ASSIGNCOPY(_accessibilityLabel, label);  // copies
}

This prevents potential bugs where a caller passes an NSMutableString, then mutates it after assignment.

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[rmottola]

Does this make sens at all at a first glance? parameters are NSStrings and not mutable, aren't they? is there a deeper issue?