fix: parse PUSHER_USE_SSL and PUSHER_DISABLED env booleans correctly

[SueJianjian]

Summary

• parse PUSHER_USE_SSL and PUSHER_DISABLED using explicit boolean coercion instead of Python truthiness
• preserve existing behavior for already-boolean values
• add focused regression coverage for string env inputs

Closes #6582

Testing

• .venv313\Scripts\python.exe -m pytest tests/test_dependencies.py -q

[CLAassistant]

All committers have signed the CLA.

[shahargl]

I think this is a bit over engineering, you can use Config class for that

[SueJianjian]

Updated this to use the existing Config bool parsing instead of a custom helper, and switched the tests to cover get_pusher_client() behavior directly. Could you take another look when you have a moment?