Positron Notebooks - Support for `%%javascript` magic command

System details:

Positron and OS details:

Session details:

Python 3.13.5

Describe the issue:

In Jupyter, VS Code, and OLD notebooks, %%javascript magic commands used to work. In new notebooks, they don't work.

Steps to reproduce the issue:

1. In a notebook with a Python kernel, attempt to run the following magic command in a cell.

%%javascript
const div = document.createElement('div');
div.style.cssText = `
    width: 100px; height: 100px; 
    background: linear-gradient(45deg, #ff6b6b, #4ecdc4);
    border-radius: 50%; margin: 20px auto;
    animation: pulse 2s infinite;
`;
div.innerHTML = '<div style="text-align:center;line-height:100px;color:white;font-weight:bold;">✨ Positron ✨</div>';
const style = document.createElement('style');
style.textContent = '@keyframes pulse { 0%, 100% { transform: scale(1); } 50% { transform: scale(1.2); } }';
document.head.appendChild(style);
element.append(div);

2. Notice that the output is an error about not being able to handle mime types.

Expected or desired behavior:

Javascript runs and pulsating circle with gradient coloring and message ('✨ Positron ✨') is displayed in the middle.

Start by reproducing the %%javascript cell in a new notebook with a Python 3.13.5 kernel and inspect the MIME-type handling path that produces the reported error. Done means the JavaScript executes and the pulsating gradient Positron message is displayed in the notebook output.