Regression - Moving multiple bezier keys in the regular track editor prints "Index (uint32_t)p_index = 1 is out of bounds (bt->values.size() = 1)." error

### Tested versions

Reproducible in: Master (3935fbca7b)
Not reproducible in: 4.7.2.stable
Bisected to: 885d4ef43f7db3f89a5c77690923c8b4c1cce7f6

### System information

Godot v4.8.dev (885d4ef43) - Windows 11 (build 26200) - Multi-window, 1 monitor - Direct3D 12 (Forward+) - dedicated NVIDIA GeForce RTX 4070 Laptop GPU (NVIDIA; 32.0.15.8129) - AMD Ryzen 7 7840HS w/ Radeon 780M Graphics (16 threads) - 15.18 GiB memory - WASAPI (48000 Hz, Stereo/mono)

### Issue description

When moving multiple bezier keys in the regular editor an error prints:

```
 ERROR: scene\resources\animation.cpp:3495 - Index (uint32_t)p_index = 1 is out of bounds (bt->values.size() = 1).
```

I bisected the issue and found it was introduced in https://github.com/godotengine/godot/pull/122667

### Steps to reproduce

https://github.com/user-attachments/assets/2a31db0e-c1bb-4cc2-947e-faf6e77ae1da

### Minimal reproduction project (MRP)

NA

Reproduce the error by moving multiple bezier keys in the regular track editor, then inspect scene/resources/animation.cpp around line 3495 and the changes from PR 122667. Compare the behavior with 4.7.2.stable; the work is done when moving multiple bezier keys no longer prints the out-of-bounds error.

Written by the indexing model from the issue text.