CodeBox: auto-install does not work — import of missing packages raises ModuleNotFoundError

## Description

The documentation claims CodeBox automatically detects and installs missing packages (`code-execution.mdx` Step 2, `quickstart-python.mdx`, `core-concepts.mdx`). In practice, this does not work — importing a package not present in `python:slim` raises `ModuleNotFoundError`.

## Reproduction

```python
import asyncio
import boxlite

async def main():
async with boxlite.CodeBox() as codebox:
result = await codebox.run("""
import requests
response = requests.get('https://api.github.com/zen')
print(response.text)
""")
print(result)

asyncio.run(main())
```

## Expected

CodeBox detects `import requests`, installs `requests` via pip, then re-runs the code.

## Actual

```
ModuleNotFoundError: No module named 'requests'
```

## Environment

- boxlite 0.5.10
- macOS Apple Silicon
- Python 3.12

## Notes

`install_package("requests")` works fine when called explicitly. The issue is specifically the auto-detection + auto-install feature.