Add threadId to aiMessages GraphQL query response

Problem

When loading existing messages in Duo Chat via the getAiMessages query, we need access to the thread ID to maintain conversation context. Currently, the getAiMessages query doesn't return thread ID information, which prevents proper thread management for existing conversations.

Current Query

query getAiMessages {
 aiMessages {
   nodes {
     id
     requestId
     content
     contentHtml
     errors
     role
     timestamp
     extras {
       sources
       hasFeedback
     }
   }
 }
}

## Desired Query

```graphql
query getAiMessages {
  aiMessages {
    nodes {
      id
      requestId
      content
      contentHtml
      errors
      role
      timestamp
      threadId
      extras {
        sources
        hasFeedback
      }
    }
  }
}

Implementation Details

Add threadId field to the AiMessage nodes in the query response Field should be of type AiConversationThreadID to match the existing chat mutation schema Should return the same thread ID format as used in the chat mutation: gid://gitlab/Ai::Conversation::Thread/{id}

Why

• Frontend needs thread context when loading existing conversations
• Required for proper thread management in multi-threaded chat UI
• Enables continuity of conversations in existing threads