Light2Ds do not affect CanvasItems that are inside their same CanvasLayer

### Tested versions

- Reproducible in 4.3
- Reproducible in 4.2.2

### System information

Windows 10

### Issue description

There's a possibility this is a misunderstanding, an intentional limitation, but I'm doubtful. There's no mention of this in the documentation.

This is reproducible on **all** rendering methods.

In short, 2D lights do not work **at all** on **CanvasItem**s that are children of a **CanvasLayer** node, even though the lights are inside the same **CanvasLayer**:

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It also means that CanvasItemMaterial's `light_mode` is entirely useless outside in these cases:

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Very importantly, note that **Light2D**s inside a **CanvasLayer** **DO** work:

### Steps to reproduce

Multiple ways, but the simplest is to create a **CanvasLayer** and insert a **Sprite2D** and a **PointLight2D** as children. Make sure to include textures for both.

### Minimal reproduction project (MRP)

[mrp-canvaslayer-light2d-do-not-work.zip](https://github.com/user-attachments/files/16796973/mrp-canvaslayer-light2d-do-not-work.zip)

Start by running the attached minimal reproduction project and verify the behavior with a CanvasLayer containing a Sprite2D and PointLight2D, across the rendering methods mentioned. Trace the CanvasLayer, CanvasItem, and Light2D rendering path to determine whether this is an implementation bug or an intentional limitation. Done means the reported lighting behavior is corrected, or the limitation is documented.

Written by the indexing model from the issue text.