fix(ai-assistant): clear LLM history on restart chat

[akashrai2003]

## Description
Fixes the restart chat functionality to properly clear LLM message history by calling the library's built-in `restartChat` function and forcing component remount.

## Problem
When "Restart Chat" is clicked, the UI resets but LLM providers still receive old messages, wasting tokens and increasing costs.

## Solution
- Call `restartChat()` from `@openassistant/core` to clear library state
- Force `AiAssistant` component remount using React key prop
- Ensures both library and component state are reset

## Changes
- Added `restartChat` from useAssistant hook
- Store in ref to access latest version
- Call `restartChat()` in onRestartAssistant handler
- Added `restartKey` state to force component remount
- Applied `key={restartKey}` to AiAssistant component

## Testing
- [x] Tested locally
- [x] Verified network logs show no old messages after restart
- [x] Works with all LLM providers (OpenAI, Google Gemini)
- [x] No breaking changes

Fixes #3261

## Checklist
- [x] Code follows project style guidelines
- [x] Comments explain the fix
- [x] Commit includes DCO sign-off
- [x] No breaking changes

[Copilot]

Pull request overview

This PR fixes the restart chat functionality in the AI Assistant to properly clear LLM message history. Previously, when users clicked "Restart Chat", the UI would reset but LLM providers still received old messages in subsequent requests, wasting tokens and increasing costs.

Key Changes:

• Integrated the restartChat() function from the @openassistant/core library to clear internal library state
• Implemented component remounting using React's key prop to ensure complete state reset
• Added error handling for the restart operation with proper async/await pattern

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[Copilot]

The useEffect hook is unnecessary for updating the ref. Following React best practices and the pattern used elsewhere in the codebase (e.g., monaco-editor.tsx), refs can be updated via direct assignment in the component body. Replace the useEffect with direct assignment.

Suggested change

	useEffect(() => {
	restartChatRef.current = libraryRestartChat;
	}, [libraryRestartChat]);
	restartChatRef.current = libraryRestartChat;

[lixun910]

Thank you @akashrai2003 !