Refactor stickman rig and ragdoll mechanics; enhance pose restoration and stability

- Disabled constraints for the stickman rig's head bone to allow for smoother animations.
- Improved mode transitions in the sandbox stage to preserve stickman poses between EDIT and DIRECT modes.
- Added side profile stabilization for knee bends in the ragdoll component to prevent unnatural poses during physics interactions.
- Enhanced the snap-to-standing functionality to restore the stickman's original pose accurately.
- Introduced new tests for validating pose preservation during mode transitions and ragdoll behavior during walking animations.
- Created a new scene for testing stickman bending and ragdoll transitions.

scenes/stickman_rig.tscn

@@ -57,11 +57,7 @@ joint_two_bone2d_node = NodePath("Torso/LeftUpperLeg/LeftLowerLeg")
 bone_index = 1
 bone2d_node = NodePath("Torso/Head")
 target_nodepath = NodePath("../IK_Targets/Head")
-enable_constraint = true
-constraint_angle_min = -180.0
-constraint_angle_max = 180.0
-constraint_angle_invert = false
-constraint_in_localspace = false
+enable_constraint = false
 
 [sub_resource type="SkeletonModificationStack2D" id="SkeletonModificationStack2D_j4hao"]
 resource_local_to_scene = true