feat(outputs): detect and inject plotly.js for text/html outputs

[rgbkrk]

Context

When Plotly uses non-default renderers (notebook, iframe_connected), it emits text/html output containing <script> tags that reference window.Plotly or require('plotly'). Currently, plotly.js is only injected into iframes when application/vnd.plotly.v1+json is detected in the output MIME types.

This means text/html Plotly outputs from these backends silently fail — the scripts execute but window.Plotly is undefined.

Possible Approach

Scan text/html output content for known Plotly signatures (e.g., Plotly.newPlot, require(['plotly'], require("plotly")) before rendering. If detected, inject plotly.js into the iframe before the HTML is rendered.

This would be similar to how getRequiredLibraries() scans MIME types today, but extended to peek at HTML content for known library references.

Notes

• The default plotly_mimetype renderer outputs application/vnd.plotly.v1+json which works correctly via the renderer plugin API
• Self-contained HTML outputs (e.g., Plotly's cdn renderer which includes a CDN <script> tag) already work
• The gap only affects renderers that expect plotly.js to be pre-loaded
• Related: Support plotly's application/vnd.plotly.v1+json MIME type (plotly_mimetype renderer) #1255 (original plotly MIME type support)