Duplicate case-sensitivity logic and `mode` string in `ExactMatch` and `Contains`

[deyna256]

ExactMatch and Contains share identical logic in two places:

1. validate() — case-insensitive comparison pattern:

if self.case_sensitive:
    passed = response == self.expected   # or: self.substring in response
else:
    passed = response.lower() == self.expected.lower()

This block is copy-pasted verbatim in both classes.

2. describe() — mode label string:

mode = "case-sensitive" if self.case_sensitive else "case-insensitive"

This expression is identical in both ExactMatch.describe() and Contains.describe().

Any change to the case-insensitive comparison logic or the mode label must be applied in two separate places.