ci: add pull request build workflow for the project

[plexoos]

This change introduces a GitHub Actions workflow to automatically build and test pull requests against all branches. The workflow runs on Ubuntu and validates code quality across multiple Python versions (3.12–3.14).

Key features:
• Runs the test suite on every pull request using a build matrix of Python 3.12–3.14.
• Installs project dependencies and common developer tools (pytest, pytest-cov, mypy, types-requests).
• Performs type checking on the src directory with mypy.
• Executes tests with coverage reporting and posts a summary as a pull request comment (for Python 3.14 job).
• Uses concurrency control to cancel in-progress builds for the same PR.

Additional updates:
• Updated pyproject.toml to silence missing imports for the stomp.* module.
• Added an empty init.py in src/swf_common_lib/ to mark the directory as a package.

[github-actions]

Test Coverage Summary

Name                                  Stmts   Miss  Cover   Missing
-------------------------------------------------------------------
src/swf_common_lib/__init__.py            0      0   100%
src/swf_common_lib/api_utils.py          33     33     0%   8-80
src/swf_common_lib/base_agent.py        295    295     0%   5-538
src/swf_common_lib/logging_utils.py      33      0   100%
src/swf_common_lib/rest_logging.py       76     76     0%   8-157
-------------------------------------------------------------------
TOTAL                                   437    404     8%
============================== 5 passed in 0.23s ===============================