Allow plugins to reference code inside .ipynb code cells

[spnotold]

Summary

Codex plugins currently cannot reference code inside code cells / code blocks from .ipynb notebooks.

Problem

When working with a Jupyter notebook, code that appears inside notebook code cells is not available to Codex plugins as a normal referencable code block. As a result, plugin workflows that need to inspect, cite, or operate on code from an .ipynb file fail or cannot point back to the relevant code.

Expected behavior

Codex plugins should be able to reference code contained in .ipynb code cells in the same way they can reference code from regular source files or Markdown fenced code blocks.

Actual behavior

The plugin cannot reference the code inside the notebook code block / code cell, so the workflow loses the relevant notebook code context.

Reproduction outline

1. Open or provide an .ipynb file that contains at least one code cell.
2. Ask a Codex plugin workflow to inspect or reference code from that notebook cell.
3. Observe that the plugin cannot reference the notebook cell code as a normal code block.

Impact

This makes Codex plugin workflows unreliable for notebook-heavy projects, especially when the user needs the plugin to inspect, cite, or modify code contained in .ipynb cells.