Format a 10-digit phone number string into the pattern (XXX) XXX-XXXX. Return the original string if it's not exactly 10 digits.

Examples:

Input: phoneNumber = "1234567890" Output: "(123) 456-7890" Explanation: Formatted 10-digit number with area code in parentheses and hyphen Input: phoneNumber = "123456789" Output: "123456789" Explanation: Not 10 digits, so returned unchanged
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