fix(camera): activate the spring arm with the third-person rig

Real defect, found by reading the code rather than by the harness: the spring arm was
created with SetAutoActivate(false), and ToggleCamera only activated the camera. A
deactivated spring arm does not tick, and its collision probe runs in that tick, so the
boom could never shorten no matter what stood behind the pawn. ToggleCamera and BeginPlay
now activate the arm alongside the camera.

The harness that was meant to prove the behaviour is still not working, and that is
recorded rather than glossed: -cameratest spawns a wall behind the pawn, but neither a
spawned static-mesh actor nor a runtime blocking volume blocks a trace (both report
blocks trace=0), so every 'arm 340 of 340' line so far was measured with no wall present
and is not evidence either way.

Game/GigLine/Source/GigLine/GigLineCharacter.h

@@ -207,6 +207,9 @@ protected:
 	/** `-cameratest`: back the pawn against a wall and check the boom does not clip. */
 	void UpdateCameraCollisionTest(float DeltaSeconds);
 
+	UPROPERTY()
+	TObjectPtr<class ABlockingVolume> CameraTestWall;
+
 	FVector LastWallNormal = FVector::ZeroVector;
 	bool bCameraTest = false;
 	bool CameraTestPlaced = false;