[RTE] does not escape HTML-like input, corrupting content with angle-bracket characters

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Target branch: `unstable`

Observed behavior

The Rich Text Editor (RTE) does not escape HTML-like input entered as plain text. When a user types a string such as `

` or `<>` in an RTE field:

1. While editing, the text appears as typed.
2. After closing the editor (switching to view mode), the content is rendered as HTML — the tag is interpreted by the browser, causing the literal text (e.g. ``) to disappear from the rendered output.

This affects languages that use angle-bracket guillemets for quotes (e.g. Spanish writes `<>`). After closing the editor, `<>` collapses to `<>` because `` is parsed as an HTML tag.

Additionally, when attempting to save or publish a QTI package containing `` (or any other invalid HTML tag) in the content, the backend raises an error at publish time because `` is not a valid tag.

Errors and logs

No specific logs available at this time.

Expected behavior

Any HTML-like text typed into an RTE field should be treated as plain text and properly escaped (e.g. `` should be stored and displayed as the literal string ``, not interpreted as an HTML tag). Angle-bracket guillemets used in languages like Spanish (`<>`) should survive close/reopen cycles without being mangled.

User-facing consequences

• Content is silently corrupted: text containing angle-bracket characters disappears from view after closing the editor, with no warning to the user.
• Users writing in languages that use angle-bracket guillemets (e.g. Spanish) cannot reliably enter quoted text in RTE fields.
• Attempting to publish a QTI package containing such content results in a backend error, blocking the publish workflow entirely.

Steps to reproduce

1. Open any RTE field in Studio (e.g. a question body in an assessment).
2. Type a string containing angle brackets, such as `` or `<>`.
3. Close the editor (switch to view mode).
4. Observe that the angle-bracket text is rendered as HTML and disappears from the displayed content.
5. (For the publish error) Save the content and attempt to publish a QTI package — the backend returns an error because the invalid tag is present in the stored content.

Context

• Affects all browsers and operating systems.
• Observed on the `unstable` branch.

Acceptance Criteria

• HTML special characters typed as plain text in any RTE field are escaped on save/close, and displayed as literal text in view mode.
• Angle-bracket guillemets (e.g. `<>`) survive close/reopen cycles without being mangled or collapsed.
• QTI packages containing angle-bracket characters in content can be published without backend errors.
• No data loss: existing content with angle-bracket characters is not silently corrupted.

AI usage

Drafted with Claude Code. Reproduction steps and acceptance criteria were reviewed and confirmed manually.