Find Valid Pair of Adjacent Digits in String

Description

You are given a string s consisting only of digits. A valid pair is defined as two adjacent digits in s such that:

- The first digit is not equal to the second.

- Each digit in the pair appears in s exactly as many times as its numeric value.

Return the first valid pair found in the string s when traversing from left to right. If no valid pair exists, return an empty string.