Motivation

• Replace the old regex assertion with clearer, focused APIs for string regex checks and centralize pattern validation in a helper.
• Provide both a vector-level assertion (assert_all_strings_contain) and a string-level assertion (assert_string_contains) that reuse the same validation logic.

Description

• Add contains_pattern() helper that validates pattern and grepl flags and returns whether all elements match the regex.
• Implement assert_all_strings_contain() as a chain of assert_character_vector, assert_no_missing, and an assert_create() wrapper around contains_pattern with a default error message.
• Implement assert_string_contains() as assert_create_chain(assert_string, assert_all_strings_contain) so single-string checks reuse the vector-level logic.
• Remove the old assert_matches_regex manpage and update NEWS.md, NAMESPACE, and tests to reflect the new assertion names and behavior.

Testing

• Ran devtools::document() which regenerated Rd files and updated NAMESPACE successfully.
• Ran devtools::test() and unit tests passed after adding coverage for assert_string_contains (tests report all tests passing).
• Ran devtools::check() which completed but returned 1 WARNING (missing qpdf for PDF size checks) and 1 NOTE (unable to verify current time), causing a non-zero exit status; there are no test failures reported.

Codex Task