You are given a string composed entirely of ‘(’ and ‘)’ characters. Your goal is to identify the longest contiguous segment (substring) within this string that represents a “well-formed” or “valid” sequence of parentheses.

A substring is considered valid if:

1. Every opening parenthesis ‘(’ has a corresponding closing parenthesis ‘)’.

2. The pairs of parentheses are correctly nested.

Return the length of this longest valid substring.

Constraints:

• $0 \leq$ s.length $\leq 3 \times 10^3$ ...