[RTE] Formula nodes do not show selection highlight

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Observed behavior

When selecting text in the RTE that includes a formula node, the formula node does not display a selection highlight. The surrounding text shows the expected blue highlight, but the formula node appears visually unselected — even though the formula is actually included in the selection and can be copied.

Expected behavior

Formula nodes should display a selection highlight when included in a text selection, consistent with the visual behavior of the surrounding text.

User-facing consequences

Users may think they cannot copy-paste already inserted formulas, leading to frustration since formulas are difficult to write. This could cause users to unnecessarily re-type complex formulas instead of reusing them.

Steps to reproduce

1. Open an exercise node in Studio and edit a question that contains a formula.
2. Click and drag to select text that includes the formula node.
3. Observe that the surrounding text shows a blue selection highlight, but the formula node does not.
4. Note that despite the missing highlight, the formula is included in the selection and can be copied and pasted.

AI usage

Used Claude to help draft this issue from a description and screenshot. Reviewed and confirmed all sections for accuracy.

Start by reproducing the selection behavior in the Studio exercise editor's RTE with text that includes a formula node. Compare the formula node's appearance with surrounding selected text and verify that the formula remains included when copied; done means the node visibly highlights consistently with the selection.