fix: resolve Flask context crashes, JSON serialization, and update visibility tests

- Replaced current_app.logger with standard logging in decision_controller.py and visibility_control_core.py to prevent startup crashes caused by missing application context.
- Fixed double JSON serialization in API discovery requests by switching from json= to data= when using CustomJSONEncoder.
- Updated Robot Framework visibility tests to expect a 404 ProblemDetails response instead of a 200 OK when visibility rules hide all APIs.