-----Description----- 
This task requires writing a Lean 4 method that determines whether a given string contains the character 'z' or 'Z'. The method should return true if the string includes either the lowercase or uppercase letter 'z', and false otherwise.

-----Input-----
The input consists of:
s: A string.

-----Output-----
The output is a Boolean value:
Returns true if the input string contains the character 'z' or 'Z'.
Returns false if the input string does not contain the character 'z' or 'Z'.

-----Note-----
There are no preconditions; the method will always work as strings and sequences are considered non-null.