Enhance health check workflows and scripts

[KevinMind]

Fixes: mozilla/addons#15464
Fixes: mozilla/addons#15450

Description

• Added output for fork detection in health_check_completed.yml.
• Updated health_check.yml to ensure health check file existence before proceeding.
• Refactored health_check.py to use a session with retries for HTTP requests, improving error handling.
• Modified tests in test_health_check.py to validate new error handling and retry logic.
• Added dummy_monitor to trigger successful or failing monitors in any environment

Context

This is patching a few holes at once but its a nightmare to test and they all cover the originally discovered problem

Testing

Test failing healtcheck

• make up the env to ensure the waffle switch is created
• run the healtcheck which should pass

./scripts/health_check.py --env local --verbose --retries 0

• Enable the waffle switch to trigger a failure

./manage.py waffle_switch dummy-monitor-fails on

• Rerun the healthcheck and now it should fail

Note

If you're feeling very adventurous you could add a high retry count and disable the waffle switch midway.. should be failures until you disable it

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.