[Feature] Support regex as a stopping condition

[mickqian]

Motivation

Addressing the issue #2007.

Modifications

1. Adds stop_regex_strs field to SamplingParams
2. Implement stop logic in Req::check_finished

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.

This PR is based on top of #2035, with modifying the stopping logic.