Smallest Covering Substring

You are given two strings: a source string s and a pattern string t. Your task is to find the shortest contiguous segment within s that contains every character present in t, accounting for their exact frequencies (including duplicate characters).

If no such segment exists in s that satisfies this requirement, return an empty string "".

A contiguous segment (or substring) is defined as a sequence of consecutive characters extracted from the original string, preserving their relative order. For instance, in the string "HELLO", valid substrings include "EL", "LLO", and "HELLO" itself, but "HLO" is not a valid substring because the characters are not consecutive in the original string.

The problem guarantees that if a valid answer exists, it will be unique—there will be exactly one shortest substring covering all characters of t.

Important Clarifications:

• The covering substring must contain at least the required number of each character from t. It may contain additional characters beyond those in t.
• Characters in both strings are case-sensitive. For example, 'A' and 'a' are treated as different characters.
• The substring must include each character from t with at least its corresponding frequency. If t = "AAB", any valid covering substring in s must contain at least two 'A's and one 'B'.