Deserializing custom resource that contains a PackedScene with a custom class script attached results in parser error

### Tested versions

Reproducible in 4.4 rc1

### System information

Godot v4.4.rc1 - Windows 11 (build 26100) - Multi-window, 2 monitors - Vulkan (Forward+) - dedicated AMD Radeon RX 5700 XT (Advanced Micro Devices, Inc.; 32.0.12033.1030) - AMD Ryzen 9 7900X 12-Core Processor (24 threads)

### Issue description

Closely related to #102551, and not fixed by #102737. Custom resource that contains a PackedScene `other`, and where `other` has an attached script `other.gd` that defines a custom class.

### Steps to reproduce

1. Open the MRP.
2. Run the main scene.
3. Note error.

### Minimal reproduction project (MRP)

[mrp.zip](https://github.com/user-attachments/files/18953197/mrp.zip)

Open the attached mrp.zip, run the main scene, and reproduce the parser error described in the issue. Read the related issues #102551 and #102737 for prior investigation, then trace the custom resource and its contained PackedScene with the attached custom-class script. Done means the minimal reproduction runs without the parser error.

Written by the indexing model from the issue text.