jupyterlab-myst fails to eval

## Description

I have installed jupyterlab-myst in my own JupyterLite deployment (https://lab.climet.eu/main). The frontend-only functionality works, e.g. I can render extra Markdown as expected. jupyterlab-myst also offers in-Markdown evaluation using the

```
{eval}`1+2`
```

syntax which calls into the running kernel. In JupyterLite, this does not seem to work. Instead of displaying `3`, it shows `1+2`. In the debug console log, I can see that communication with the kernel occurs:

```
Executing cell, expressions undefined [956.61770f21fa231b6605ff.js:2:16493](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Markdown cell 4744c1f8-c913-4899-81e3-f546eec61f87 was executed [956.61770f21fa231b6605ff.js:2:16690](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Executing named expressions
Map { 0 → "1+2" }
[956.61770f21fa231b6605ff.js:2:17101](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Performing kernel request
Object { code: "", user_expressions: {…} }
[956.61770f21fa231b6605ff.js:2:17288](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Re-rendering VDOM for MySTWidget
Object { stateChanged: {…}, _isDisposed: false, _references: {…}, _frontmatter: {}, _mdast: {…}, _expressions: undefined }
false [956.61770f21fa231b6605ff.js:2:7570](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Re-rendering VDOM for MySTWidget
Object { stateChanged: {…}, _isDisposed: false, _references: {…}, _frontmatter: undefined, _mdast: {…}, _expressions: undefined }
false [956.61770f21fa231b6605ff.js:2:7570](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Re-rendering VDOM for MySTWidget
Object { stateChanged: {…}, _isDisposed: false, _references: {…}, _frontmatter: undefined, _mdast: {…}, _expressions: undefined }
false [956.61770f21fa231b6605ff.js:2:7570](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Re-rendering VDOM for MySTWidget
Object { stateChanged: {…}, _isDisposed: false, _references: {…}, _frontmatter: undefined, _mdast: {…}, _expressions: undefined }
false [956.61770f21fa231b6605ff.js:2:7570](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Handling kernel response
Object { channel: "shell", header: {…}, parent_header: {…}, metadata: {}, content: Object { payload: [], status: "ok", execution_count: 1 }, buffers: [] }
[956.61770f21fa231b6605ff.js:2:17368](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Handling evaluated user expressions
Array []
[956.61770f21fa231b6605ff.js:2:17720](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
trust changed true [956.61770f21fa231b6605ff.js:2:11769](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
Re-rendering VDOM for MySTWidget
Object { stateChanged: {…}, _isDisposed: false, _references: {…}, _frontmatter: undefined, _mdast: {…}, _expressions: undefined }
true [956.61770f21fa231b6605ff.js:2:7570](https://lab.climet.eu/main/extensions/jupyterlab-myst/static/956.61770f21fa231b6605ff.js?v=61770f21fa231b6605ff)
```

When executing later code cells, the execution counter is increased, so clearly the kernel receives and executes the eval request, but for some reason does not reply with the result.

## Reproduce

1. Go to https://lab.climet.eu/main
2. Execute a Markdown cell with content "{eval}\`1+2\`"

## Context

- JupyterLite version: v0.7.0
- Operating System and version: macOS 26.2
- Browser and version: Firefox 146.0.1

Reproduce the issue at https://lab.climet.eu/main using a Markdown cell containing `{eval}`1+2` and inspect the kernel request and response shown in the debug console. Compare the evaluated user expressions with the expected result; done means the cell displays `3` in JupyterLite rather than `1+2`.

Written by the indexing model from the issue text.