AnimationNodeTransition transition_to_self doesnt fade when transitioning to self.

### Tested versions

- reproducible in godot v4.3 (stable)

### System information

Windows 10 - Godot 4.3 (Stable) - Vulkan Forward+

### Issue description

When you make a transition node in an AnimationTree and set it to transition to self it does not fade, instead simply starting the animation over entirely unless you create a duplicate animation input.

This could be how its intended but feels wrong that when i want to fade an animation to itself on a AnimationTransitionNode i have to make a copy if that animation.

### Steps to reproduce

Create a new AnimationTree, create an AnimationNodeTransition and set to transition to self. Give it an Xfade. Have it transition to self while game is running. instead of fading to self it will simply restart the animation.

### Minimal reproduction project (MRP)

[animationtransitionnode.zip](https://github.com/user-attachments/files/17371131/animationtransitionnode.zip)

Start by running the attached animationtransitionnode.zip MRP in Godot 4.3 and reproduce the AnimationTree AnimationNodeTransition transition-to-self behavior with an Xfade. Trace the AnimationNodeTransition path involved in that transition and verify that a self-transition fades as described rather than simply restarting the animation.

Written by the indexing model from the issue text.