fix(devices): defer Aravis import-guard from Camera.__init__ to on_connect()

Camera.__init__ raised ImportError unconditionally whenever the Aravis
GObject-Introspection library wasn't importable, even with connect=False --
contradicting the module's own "lazy initialization" docstring and
breaking every unit test in CI, since Aravis is a system library (not
pip-installable) that CI never has installed. The test fixture builds a
real AlliedVisionAravisCamera(connect=False) and only mocks camera.cam
afterward, so the raise fired before mocking ever got a chance to run.

Move the guard into on_connect(), mirroring IDSCamera's working pattern
(its equivalent check lives in the object only built by on_connect(),
never at construction time). Construction is now independent of whether
Aravis is installed; on_connect() still raises the same clear error the
moment something actually tries to talk to hardware.

csaxs_bec/devices/allied_vision_cameras/base_integration/camera.py

@@ -46,12 +46,6 @@ class Camera:
     """
 
     def __init__(self, camera_id: str, connect: bool = True):
-        if Aravis is None:
-            raise ImportError(
-                "The Aravis library is not installed, or its GObject Introspection typelib"
-                " is missing. Please check your Python environment (PyGObject) and Aravis"
-                " installation."
-            )
         self.Aravis = Aravis
         self.GLib = GLib
         self.camera_id = camera_id
@@ -72,6 +66,12 @@ class Camera:
 
     def on_connect(self):
         """Connect to the camera and initialize it."""
+        if self.Aravis is None:
+            raise ImportError(
+                "The Aravis library is not installed, or its GObject Introspection typelib"
+                " is missing. Please check your Python environment (PyGObject) and Aravis"
+                " installation."
+            )
         if self._connected:
             logger.warning("Camera is already connected.")
             return