Regression? FileAccess will load files next to binary when not found in pck using res:// path

### Tested versions

Tested - v4.5.stable.official [876b29033]
Tested - v4.4.stable.official [4c311cbee]

### System information

Godot v4.5.stable - Windows 10 (build 19045) - Single-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2070 SUPER (NVIDIA; 32.0.15.7688) - Intel(R) Core(TM) i9-9900KF CPU @ 3.60GHz (16 threads) - 31.92 GiB memory

### Issue description

I believe that there is a regression in how file paths are handled. Specifically:

**How I have understood it to work previously:**
When using FileAccess to open a file at "res://test.txt", the file must be present in the PCK. The "res://" path will specifically force Godot to only look inside the PCK/Project. And an exported project will **not** load files that are actually outside the PCK, where they are actually at "./test.txt"

**How it appears to work now:**
Supposing the following file structure:

```
Folder
- game.exe
- game.pck <- Does not container text.txt
- text.txt
```

**What I think should happen:**
`FileAccess.open("res://text.txt") == FILE_NOT_FOUND`
`FileAccess.open("./text.txt") == OK` <- Fails to find in PCK first, then falls back to relative.
`FileAccess.open(ProjectSettings.globalize_path("res://text.txt")) == OK` <- Only checks relative, never PCK

**What does happen:**
`FileAccess.open("res://text.txt") == OK`
`FileAccess.open("./text.txt") == OK` <- No change.
`FileAccess.open(ProjectSettings.globalize_path("res://text.txt")) == OK` <- Now works like "./"

### I'd like a sanity check on this, because there appears to be no documentation on whether or not res:// >should or should not< allow loading external files.

If this is intentional, I'd say we make a documentation issue instead of this. Though I'd object to that being how it should work. ¯\_(ツ)_/¯

### Steps to reproduce

Refer to the examples / MRP.

### Minimal reproduction project (MRP)

[res_path_error_test.zip](https://github.com/user-attachments/files/22659847/res_path_error_test.zip)

This project is made in Godot 4.5, but it will run without changes in 4.4. Make sure to exclude test_resource.tres in the export, so that it will not be included in the .pck.

**Output in 4.5:**
As a tool, or from the editor, or exported, all three will result in the same outcome:

```
FILE TEST
PACK result = 0
RELATIVE result = 0
GLOBALIZED result = 0
RESOURCE TEST
PACK result =
RELATIVE result =
GLOBALIZED result =
```

Expected behavior: Both PACK results fail to open/load the file.

Start with the attached res_path_error_test.zip MRP and reproduce the exported-project results after excluding test_resource.tres from the PCK. Trace the FileAccess.open("res://text.txt") and ProjectSettings.globalize_path("res://text.txt") entry points, then compare their behavior with ./text.txt. Done means the intended distinction between PCK-only and external-file loading is established and the regression or documentation gap is addressed.

Written by the indexing model from the issue text.