Add Python import validation to CI pipeline

[dandye]

Summary

• Adds GitHub Actions workflow to catch Python import errors before they reach production
• Implements pragmatic file-by-file import testing to prevent errors like from typing import str
• Tests all Python files across server directories to ensure they can be imported without errors

Context

A production incident occurred when from typing import str (an invalid import) made it through to deployment. Standard tools like python -m compileall only catch syntax errors, not import errors. This PR implements a practical solution to catch these errors in CI.

Implementation

• .github/workflows/pr-tests.yml: GitHub Actions workflow that runs on all PRs
• tests/check_imports.py: Script that attempts to import every Python file to catch runtime import errors

Caveats & Trade-offs

This is an expedient solution with known limitations:

Known Issues

• Brute force approach: Tests every .py file in isolation, including setup.py and test files
• Potential false positives: Some files may not be designed to run standalone
• Performance: Importing every file individually is slower than ideal
• No exclusion mechanism: Cannot skip specific files or patterns
• Missing dependencies: Files might fail if they import optional dependencies

Why This Approach

• Catches the exact error that broke production (from typing import str)
• Simple and pragmatic: No complex configuration or type annotation requirements
• Works immediately: Doesn't require fixing legacy code style issues
• Better than alternatives:
  • compileall doesn't catch import errors
  • mypy would require type hints throughout legacy code
  • Linters (ruff, flake8) don't catch invalid typing imports

Future Improvements

Consider these enhancements in follow-up PRs:

• Add exclusion patterns for test files and setup.py
• Implement parallel testing for performance
• Add configuration file for customizing behavior
• Consider migrating to mypy once legacy code is cleaned up

Test Plan

• Tested locally - correctly identifies import errors with exit code 1
• Verified it catches the specific from typing import str error
• CI workflow will validate on this PR

This is a pragmatic, immediate fix to prevent critical import errors from reaching production. While not perfect, it solves the immediate problem and can be refined over time.