[codex] restore bqplot 0.13 lab 3 visual coverage

[maartenbreddels]

Summary

This is a follow-up to the bqplot 0.12/0.13 compatibility work that was merged before the final CI cleanup landed.

The previous PR added bqplot 0.13 coverage but temporarily skipped the JupyterLab runner for that environment while we worked through the runtime/package split. That left the bqplot 0.13 visual job testing Solara and classic Notebook, but not the historical JupyterLab path.

Details

This restores JupyterLab coverage for the bqplot 0.13 visual environment while keeping it on JupyterLab 3. That matches the JupyterLab major version the project has historically tested, and avoids mixing the current compatibility PR with the separate JupyterLab 4 federated-extension problem.

The build-system dependency is also pinned to jupyterlab>=3.6,<4. Without the upper bound, isolated builds can pull JupyterLab 4 to build the labextension metadata, even when the test environment itself is running JupyterLab 3. That mismatch produced confusing shared-package warnings in the browser and made it look like the runtime was testing one thing while the built bundle expected another.

I also clarified the bqplot-gl loader error messages. The classic loader now distinguishes between “there is no RequireJS loader” and “bqplot-gl is missing from the classic frontend”, while the JupyterLab loader message no longer tells users to enable an nbextension.

Validation

• npm run build passed using JupyterLab 3.5.2 on PATH.
• Earlier local Docker checks showed the bqplot 0.13 JupyterLab runner renders under JupyterLab 3; the remaining image mismatch was from the existing JupyterLab news/privacy toast baseline and was intentionally not included in this minimized follow-up.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.81%. Comparing base (3ab3742) to head (c2b23ad).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   39.81%   39.81%           
=======================================
  Files           8        8           
  Lines         211      211           
=======================================
  Hits           84       84           
  Misses        127      127           

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[astrofrog]

@maartenbreddels - thanks! should we wait until bqplot-gl is released so we can cleanup the reference to your bqplot-gl fork, or should we merge this and release quickly?