feat: list[str] + case-insensitive support for assert_final_output(contains=...)

[Neohu-ceo]

Summary

Extends assert_final_output(contains=...) with backward-compatible features (fixes #2):

1. list[str] — contains now accepts a list; all substrings must match
2. case_sensitive=False — case-insensitive substring matching

Examples

# All substrings must be present
assert_final_output(result, contains=["Tokyo", "22°C"])

# Case-insensitive
assert_final_output(result, contains="tokyo", case_sensitive=False)

equals and matches unchanged. Default case_sensitive=True — existing tests unaffected.

[simukappu]

Thanks for the PR, and for picking up a good first issue! The implementation looks correct and is backward-compatible. Single-string input with the default case_sensitive=True keeps the existing behavior and error message.

CI is failing on the coverage check, which points at the missing tests. The new branches (the case_sensitive path and the missing / single-vs-multiple formatting) aren't exercised yet. Could you add tests under tests/test_assertions.py covering: single string (existing), list with all present, list with one missing, case-insensitive hit, and case-insensitive miss? Once those are in and CI is green, this is good to merge.

No need to touch the CHANGELOG. I'll handle that on the maintainer side. Thanks again!