Case-insensitive and multi-substring matching for `assert_final_output(contains=...)`

[simukappu]

assert_final_output(result, contains="Tokyo") currently does a case-sensitive substring check against a single string. Two small, backward-compatible additions would make it more useful:

1. Accept a list of substrings for contains, all of which must be present.
2. Add a case_sensitive: bool = True parameter that, when False, lowercases both sides before comparing.

Example

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

# case-insensitive substring check
assert_final_output(result, contains="tokyo", case_sensitive=False)

Acceptance criteria

• contains accepts both a str (existing behavior) and a list[str] (all must match).
• case_sensitive defaults to True, so existing tests are unaffected.
• The FinalOutputError message names which substring was missing when a list is passed.
• New tests cover: single string (existing), list with all present, list with one missing, case-insensitive hit, case-insensitive miss.
• equals and matches behavior is unchanged.