Exported Array default is read-only when not serialized

### Tested versions

- Reproducible: v4.6.beta2.official [551ce8d47], v4.5.1.stable.official [f62fdbde1]

### System information

Godot v4.6.beta2 - macOS Sequoia (15.5.0) - Multi-window, 2 monitors - Metal (Mobile) - integrated Apple M2 Max (Apple8) - Apple M2 Max (12 threads) - 64.00 GiB memory

### Issue description

When a Resource has an exported typed Array with a default value (e.g. @export var arr: Array[String] = ["a"]), and that property is not explicitly serialized into the .tres, loading the resource from disk makes the array read_only (is_read_only() == true).
The same resource created via ResourceA.new() yields is_read_only() == false. This difference breaks parity between “default value in script” and “default value used by a loaded resource”.

### Steps to reproduce

1. Open the project.
2. Right-click res://validator.gd and select Run.
3. Check the Output.

### Minimal reproduction project (MRP)

[array-readonly.zip](https://github.com/user-attachments/files/24343262/array-readonly.zip)

Start with the minimal reproduction project and run res://validator.gd as described, comparing the read-only state of the newly created resource with the resource loaded from disk. Trace the exported typed Array default through resource loading and serialization. Done means an unserialized default Array has the same writable behavior after loading as it does from Resource.new().

Written by the indexing model from the issue text.