Refactor auth tests to use real communities, remove mocks (#255)

* Refactor auth tests to use real communities, remove mocks

Replace MagicMock and @patch with real community configurations
loaded via discover_assistants(). Use monkeypatch for environment
variables (real Settings reads them). All test scenarios preserved.

Closes #85

* Address review: use dynamic config values, fixture for discovery

Derive test origins and model values from loaded community configs
instead of hardcoding them. Move discover_assistants() into a
module-scoped fixture for better error reporting. Add helper
functions for config access.

Author: neuromechanist