New markdown cells don't have `hide_code=True`

[Antyos]

Describe the bug

Creating a new markdown cell in the notebook view will create an @app.cell but doesn't specify hide_code=True like new markdown cells in the web-based editor.

Example:

@app.cell
def _():
    mo.md(r"""
    ## Test
    """)
    return

Environment

\`\`\`

marimo VS Code Extension Diagnostics

Language Server (LSP):
UV Bin: Bundled (c:\Users\Andrew.vscode\extensions\marimo-team.vscode-marimo-0.10.4-win32-x64\bundled\libs\bin\uv.exe)
UV: 0.9.26 (ee4f00362 2026-01-15) ✓
Using bundled marimo-lsp via uvx

Python Extension:
Interpreter: c:\Users\Andrew\Code\project\code.venv\Scripts\python.exe
Version: 3.13.9.final.0
Environment: VirtualEnvironment (biaxtic)

Python Language Server (ty):
Status: running ✓
Version: 0.0.15 (eb43dff06 2026-02-04)
Python: c:\Users\Andrew\Code\project\code.venv\Scripts\python.exe (3.13.9.final.0)

Ruff Language Server:
Status: running ✓
Version: 0.15.0

Extension Configuration:
Version: 0.10.4
UV integration disabled: false

System Information:
Host: desktop
IDE: Visual Studio Code
IDE version: 1.109.4
Platform: win32
Architecture: x64
Node version: v22.21.1

Common Issues:
1. If notebooks won't open:
- Check Python interpreter is selected
- Ensure marimo and pyzmq are installed
- Check 'marimo-lsp' output channel for errors
2. If features are missing:
- Ensure marimo version is >= 0.19.10
- Try reloading the window

Steps to reproduce

No response