fix(core): agent shutdown routine

[Archento]

Proposed Changes

This PR:

• fix the shutdown routine when using the mailbox
• add graceful shutdown for remaining tasks
• fixes KeyboardInterrupt again
• refactor registration loop

Linked Issues

closes: #577

Types of changes

• Bug fix (non-breaking change that fixes an issue).
• New feature added (non-breaking change that adds functionality).
• Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• Documentation update.
• Something else (e.g., tests, scripts, example, deployment, infrastructure).

Checklist

• I have read the CONTRIBUTING guide
• Checks and tests pass locally

If applicable

• I have added tests that prove my fix is effective or that my feature works
• I have added/updated the documentation (executed the script in python/scripts/generate_api_docs.py)

[jrriehl]

LGTM!