🧪 add tests for service.py initialization and demo setup

[joy7758]

🎯 What: The missing test file for service.py was created to test initialization and the demo method setup logic.
📊 Coverage: The test suite covers instantiation configurations (happy path, token character minimum checks, duplicated approver tokens checks, valid setup variables), demo static method explicit / environment keys instantiation, and success/failures on the authentication methods.
✨ Result: Enhanced the test coverage of action-gate missing service logic, preventing regressions on auth credentials passing logic.

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PR created automatically by Jules for task 2162347556792689612 started by @joy7758

[google-labs-jules]

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