Smallest Window Containing Pattern

Problem

Given two strings s and pattern, return the smallest substring of s that contains every character in pattern, including duplicates. If no such substring exists, return an empty string. Implement a function that performs this search efficiently.

Constraints

• 1 <= len(s) <= 10^5
• 1 <= len(pattern) <= 10^5
• s and pattern contain printable ASCII characters
• Return "" if no valid substring exists