Harden implementation and add more tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.23%. Comparing base (4f99140) to head (5e40c9f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #68 +/- ## ========================================== + Coverage 85.77% 90.23% +4.46% ========================================== Files 11 11 Lines 506 512 +6 ========================================== + Hits 434 462 +28 + Misses 72 50 -22 ```

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[sentry-io[bot]]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ RuntimeError: coroutine ignored GeneratorExit sam.slack in handle_message View Issue

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