Retry processing of messages until no change between runs

[stefpiatek]

closes #393

• [x] Allow env configuration of wait time for queues to empty before checking? Would be good to not wait during system tests

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.60759% with 9 lines in your changes missing coverage. Please review.

Project coverage is 83.60%. Comparing base (de8ea69) to head (6e1b93a).

Files	Patch %	Lines
cli/src/pixl_cli/_message_processing.py	85.45%	8 Missing :warning:
cli/src/pixl_cli/main.py	93.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #407 +/- ## ========================================== + Coverage 83.04% 83.60% +0.56% ========================================== Files 76 81 +5 Lines 3061 3374 +313 ========================================== + Hits 2542 2821 +279 - Misses 519 553 +34 ```

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