Bug: Inconsistent getTextContent works differently in $getRoot().getTextContent()

Lexical version: latest

## Steps To Reproduce

1. Create and add a custom paragraph node with a custom getTextContent, example:
```
getTextContent(): string {
const textContent = this.getChildren()
.map((child: LexicalNode) => child.getTextContent())
.join('');
return `~"${textContent}"`;
}
```
2. Add a OnchangePlugin, then getTextContent from $getRoot and from $getChildAtIndex

```
{
const textContent = editorState.read(() => $getRoot().getTextContent());

const textContent2 = editorState.read(() =>
$getRoot().getChildAtIndex(0).getTextContent(),
);
console.log('>>>>', textContent);
console.log('>>>>2', textContent2);

```

Link to code example:

## The current behavior
$getRoot().getTextContent() completely ignores the custom node getTextContent, while the $getRoot().getChildAtIndex(0).getTextContent() successfully returns the correct custom getTextContent.

$getRoot().getTextContent():
```
Hello, {firstName}, how are you?
```

$getRoot().getChildAtIndex(0).getTextContent():
```
~"Hello, {firstName}, how are you?"
```

## The expected behavior
In my opinion, the result should be the same.

Reproduce the discrepancy with the custom paragraph node and OnChangePlugin, comparing $getRoot().getTextContent() with $getRoot().getChildAtIndex(0).getTextContent(). Trace the TypeScript implementations of getTextContent from the root and child entry points; done means both calls honor the custom node output and regression coverage verifies the expected string.

Written by the indexing model from the issue text.