Loosen cell ID regex to match nbformat spec

[ellisonbg]

Summary

• Fixes LLMs can add broken cells that never show output #137
• Fix cell ID regex pattern to properly handle nbformat cell IDs with alphanumeric characters, underscores, and hyphens
• Ensure all notebook cells have IDs by upgrading notebooks to nbformat 4.5+ and generating IDs for cells that lack them
• Improve regex pattern specificity for file IDs (inline UUID pattern) and cell IDs (nbformat-compatible pattern)

Changes

• Updated cell ID regex from strict UUID pattern to [a-zA-Z0-9_-]+ to match nbformat specification
• Added automatic notebook upgrade to nbformat 4.5+ to ensure cell IDs are available
• Generate UUIDs for cells missing IDs in both read and write operations
• Simplified regex patterns by inlining UUID pattern for file IDs

Test plan

• Test API endpoints with various cell ID formats (UUID and alphanumeric)
• Verify notebooks without cell IDs get properly upgraded and assigned IDs
• Confirm outputs are correctly saved and retrieved with new cell ID handling

🤖 Generated with Claude Code

[ellisonbg]

A quick additional note: the part of this PR needed discussion is that because we require cell ids, we need to update all notebooks to nbformat 4.5. Both jupyter_ydoc and nbformat delete cell ids if present so we have to do this. An alternative would be to change nbformat and jupyter_ydoc to allow for cell ids even though they would need to be optional in the spec.

[dlqqq]

@ellisonbg Thank you for opening this PR! I was able to verify that this fixes #137 with Claude Code.

[ellisonbg]

Feel free to merge, I have another PR on the way that fixed kernel restarts, so we might want to wait for that one before we cut another release.

[dlqqq]

A quick additional note: the part of this PR needed discussion is that because we require cell ids, we need to update all notebooks to nbformat 4.5. Both jupyter_ydoc and nbformat delete cell ids if present so we have to do this.

Is this change invisible to end users (i.e. the upgrade has no intended side effects)? If so, then I think this is fine. If this causes any issue, the proper solution is to fix the migration logic in nbformat.

[ellisonbg]

The only visible change is that their notebook on disk will be in the 4.5 format. If they were deliberately using an older spec (almost unheard of) they will perhaps be surprised. We could emit a notification when this happens to let them know as a follow on.