[ZEPPELIN-6742] Connect the Assistant frontend to conversation REST and WebSocket APIs

[voidmatcha]

What is this PR for?

Connect the Assistant frontend to the conversation REST and WebSocket APIs. The transport provides conversation creation and retrieval, message sending, and response delivery for the UI.

This PR depends on server PR #5547, shared prerequisites #5558, and UI contracts #5561. Its temporary base contains exactly those changes. After a WebSocket disconnect, the transport shows an uncertain run state until the user reloads the saved history; it does not require a server-side running field.

What type of PR is it?

Feature

Todos

• Merge [ZEPPELIN-6733] Add AI assistant chat to notebooks (server-side) #5547, [ZEPPELIN-6738] Upgrade React to 19 and fix shared host integration #5558, and [ZEPPELIN-6741] Define shared Assistant UI contracts and message history behavior #5561 before this PR
• Rebase onto master and retarget this PR to master before merging
• Compare against the final API contract from server PR [ZEPPELIN-6733] Add AI assistant chat to notebooks (server-side) #5547
• Confirm CI results for the new base and head

What is the Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-6742

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/entities/assistant/model/assistantTransport.spec.ts

Also verify the REST and WebSocket integration against the final API contract from server PR #5547.

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 connects the frontend to the conversation REST API and WebSocket run events. After a connection loss, the user can reload saved history; the transport does not require a server-side running field.

The current diff is +1,460/-1 across four files. Excluding the 800 lines of tests, the review diff is +660/-1. There is no lockfile change.

This PR depends on #5547, #5558, and #5561. Its temporary base contains those prerequisites without the separate server fixes. Reviews can proceed against this diff; after the prerequisites merge, I will rebase and retarget the PR to master, then verify the final API contract and CI before merging.