Update CHANGES for assert_on_exception feature

Document the new assert_on_exception parameter in version 0.26.0.

This is a minor version bump (not patch) because we're adding new
functionality to the public API, even though it's fully backward
compatible with existing code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: RJPercival

CHANGES

@@ -1,3 +1,11 @@
+0.26.0
+------
+
+* Added `assert_on_exception` parameter to `RequestsMock` and `@responses.activate` decorator.
+  When set to True, assertions about unfired requests will be raised even when an exception
+  occurs in the wrapped code, providing valuable debugging context about which mocked
+  requests were or weren't called.
+
 0.25.8
 ------