Fix Ask AI conversation ID handling

[reakaleek]

Summary

Fixes a bug where the Ask AI chatbot could not remember previous messages in a conversation.

Problem

When a user asked a follow-up question like "What did I ask before?", the AI responded that it had no conversation history. This happened only on the first follow-up message after starting a new conversation.

Root cause

The bug was a conversation ID mismatch between two parts of the system:

User asks question (no conversation ID)
        │
        ▼
┌─────────────────────────────────────────────────────────┐
│  LLM Gateway generates ID "abc-123" for its ThreadId   │
└─────────────────────────────────────────────────────────┘
        │
        ▼
┌─────────────────────────────────────────────────────────┐
│  Stream Transformer generates DIFFERENT ID "xyz-789"   │
│  and sends it to the frontend                          │
└─────────────────────────────────────────────────────────┘
        │
        ▼
User sends follow-up with ID "xyz-789"
        │
        ▼
LLM Gateway looks for conversation "xyz-789" → NOT FOUND

The LLM Gateway used one ID internally, but the Stream Transformer created a different ID for the frontend. When the user continued the conversation, the IDs did not match.

Solution

We now ensure one ID is used everywhere:

1. When a new conversation starts, the gateway generates the ID
2. The gateway returns this ID in a new GeneratedConversationId field
3. The stream transformer uses this same ID when sending events to the frontend

New API design

// Simple response with one self-documenting field
public record AskAiGatewayResponse(
    Stream Stream, 
    Guid? GeneratedConversationId  // Non-null only when gateway created a new ID
);

Scenario	GeneratedConversationId
New conversation (LLM Gateway)	<new GUID>
Continuing conversation	null
Agent Builder (any)	null (it handles IDs in the stream)

Test plan

• Start a new Ask AI conversation
• Ask a follow-up question like "What did I just ask?"
• Verify the AI remembers the previous question
• Start another new conversation and verify it works again