Trie prefix match length

A documentation site holds every searchable term in a prefix tree, so the suggestion list is found by walking down from the root one typed character at a time. The list narrows with each keystroke and goes empty the moment a character has no branch to follow. The support team wants to know where a visitor's query left the tree. You get indexed_terms, the stored terms, and typed, what the visitor entered. Return the largest count of leading characters of typed that still match, meaning the largest k between 0 and the length of typed for which at least one indexed term begins with the first k characters of typed. Return 0 when the very first character already matches nothing, when typed is empty, or when the index holds no terms. Indexed terms are non empty and matching is case sensitive.