Migrate JupyterLab sidepanel extension to prebuilt package distribution

[Eliaaazzz]

Addresses #37977.

Summary

• migrate the Jupyter Sidepanel extension to the pip-based JupyterLab 4 prebuilt installation flow
• add the missing packaging metadata and CSS entrypoints required for prebuilt extension discovery
• remove deprecated jupyter labextension install/link guidance and update interactive docs to use the modern pip-based flow

What Changed

• add install.json so JupyterLab recognizes the extension as Python-managed
• add style/index.js and styleModule so stylesheet assets are bundled correctly for the prebuilt extension
• update package.json packaging globs so the new style entrypoint is included in published artifacts
• add JupyterLab prebuilt-extension classifiers to pyproject.toml
• include src/yaml/* in tsconfig.json so the YAML panel sources are compiled
• update the sidepanel extension README to document the prebuilt pip install workflow and remove unnecessary legacy jupyter lab build steps
• update the interactive Beam README to replace jupyter labextension install @jupyter-widgets/jupyterlab-manager with pip install ipywidgets

Testing

• jlpm run build:lib
• jlpm run build:prod
• jlpm jest --runInBand
• python -m pip install -e .
• jupyter labextension list
• python -m build --sdist --wheel

Context

• JupyterLab 4 favors prebuilt extensions distributed through Python packages instead of the legacy labextension install/link workflow.
• This change makes the sidepanel extension discoverable and installable through the expected Python packaging path, and aligns the docs with the supported installation flow.

Please add a meaningful description for your change here

Provide a way to keep certain images consumed by Dataflow managed transform upgrade test from getting deleted

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request modernizes the JupyterLab sidepanel extension by transitioning it to a prebuilt Python package distribution. This change simplifies the installation process for end-users by removing the need for manual build steps and aligns the project with current JupyterLab best practices for extension management.

Highlights

• JupyterLab Extension Migration: Migrated the sidepanel extension to the JupyterLab 4 prebuilt Python package distribution model.
• Packaging Metadata: Added necessary packaging metadata, CSS entrypoints, and installation instructions to support prebuilt extension discovery.
• Documentation Cleanup: Removed deprecated labextension installation guidance and simplified the setup process for users.

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[Eliaaazzz]

./gemini-review