Update health endpoint and refine test documentation

- Modified the health endpoint to return "None" for AI availability when inference is not initialized, improving clarity on system status.
- Enhanced the test documentation to include handling of skipped tests, emphasizing the need for investigation before proceeding.
- Updated test assertions to ensure proper execution order and prevent premature engine initialization.
- Refactored test cases to streamline performance testing and improve readability, removing unnecessary complexity.

These changes aim to enhance the robustness of the health check and improve the overall testing framework.

main.py

@@ -124,12 +124,13 @@ def detection_to_dto(det) -> DetectionDto:
 
 @app.get("/health")
 def health() -> HealthResponse:
+    if inference is None:
+        return HealthResponse(status="healthy", aiAvailability="None")
     try:
-        inf = get_inference()
-        status = inf.ai_availability_status
+        status = inference.ai_availability_status
         status_str = str(status).split()[0] if str(status).strip() else "None"
         error_msg = status.error_message if hasattr(status, 'error_message') else None
-        engine_type = inf.engine_name
+        engine_type = inference.engine_name
         return HealthResponse(
             status="healthy",
             aiAvailability=status_str,