[ZEPPELIN-6743] Mount an opt-in AI Assistant shell in the notebook sidebar

[voidmatcha]

What is this PR for?

Add an opt-in AI Assistant button and empty panel shell to the notebook sidebar. With ?reactAssistant=true, the Angular notebook mounts the React shell, coordinates it with the file tree and table of contents, and shares the sidebar width for resizing. The panel closes when the active note changes and shows a fallback if the React remote fails to load.

This establishes the notebook mount point for later conversation UI work. Conversation components, server transport, and message handling are outside this PR.

This PR depends on #5558 and #5561. Its temporary base contains those changes so the shell can be reviewed separately.

What type of PR is it?

Feature

Todos

• Merge [ZEPPELIN-6738] Upgrade React to 19 and fix shared host integration #5558 and [ZEPPELIN-6741] Define shared AI Assistant UI contracts and message history behavior #5561 before this PR
• Rebase onto master and retarget this PR to master before merging
• Confirm CI results

What is the Jira issue?

ZEPPELIN-6743

How should this be tested?

Run from zeppelin-web-angular/:

npm run build-project:sdk
npm run typecheck:react
npm --prefix projects/zeppelin-react test -- src/pages/AssistantWorkspace.spec.tsx
npm run test:shell -- src/app/pages/workspace/notebook/assistant src/app/pages/workspace/notebook/sidebar/sidebar.component.spec.ts src/app/services/react-feature.service.spec.ts
npm run lint
npm run build:react

In an authenticated notebook, open ?reactAssistant=true and check opening, closing, resizing, switching sidebar sections, and switching notes. Confirm that the button is absent without the flag and in view-only and revision views.

Screenshots (if appropriate)

N/A

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[voidmatcha]

This PR adds the opt-in Assistant sidebar shell and its Angular/React mount point. Conversation UI and server transport are outside this diff.

The current diff is +924/-7 across 21 files. Excluding 299 lines in *.spec.* tests, including the notebook E2E spec, the review diff is +625/-7. There is no lockfile change.

The SDK build, React typecheck, focused React and Angular tests, lint, and React production build pass locally. Webpack reports an asset-size warning.

This depends on #5558 and #5561. The temporary base contains those prerequisites so reviews can proceed now. After they merge, I will rebase this branch onto the latest master, retarget the PR to master, and confirm CI before merging.