Fix `jr:itext` to return the default `value` node without `form` attribute

### Description
The `jr:itext` function, which retrieves the `value` text of an `itext` node for a given ID, incorrectly returns the first `value` node regardless of its `form` attribute. It should prioritize the default `value` node (the one without a `form` attribute). This bug causes incorrect text output in forms, particularly in edge cases where `jr:itext` is used as a string function.

### Expected Behavior
`jr:itext` should return the text of the default `value` node (without a `form` attribute) from the `itext` structure, e.g., "This is the default text" for the following:

```xml

This is an alternative text
This is the default text

```

The function should respect the specified language (e.g., `fr`) or fall back to a default language if none is set.

### Actual Behavior
`jr:itext` returns the first `value` node’s text (e.g., "This is an alternative text" from `This is an alternative text`), ignoring the default node.

[Find related test cases here.](https://github.com/getodk/web-forms/blob/da4b22020c5bc651f1cda8f4a83a67a8a696badd/packages/xpath/test/xforms/jr-itext.test.ts#L218)

### Related Issues
- https://github.com/getodk/web-forms/pull/399#discussion_r2082551898

Start with packages/xpath/test/xforms/jr-itext.test.ts around line 218 and inspect the jr:itext implementation it exercises. Verify the language selection and fallback behavior, then ensure the default value node without a form attribute is returned and the related tests pass.

Written by the indexing model from the issue text.