FEATURE: support Chat with AI Persona via a DM (#488)

Add support for chat with AI personas

- Allow enabling chat for AI personas that have an associated user
- Add new setting `allow_chat` to AI persona to enable/disable chat
- When a message is created in a DM channel with an allowed AI persona user, schedule a reply job
- AI replies to chat messages using the persona's `max_context_posts` setting to determine context
- Store tool calls and custom prompts used to generate a chat reply on the `ChatMessageCustomPrompt` table
- Add tests for AI chat replies with tools and context

At the moment unlike posts we do not carry tool calls in the context.

No @mention support yet for ai personas in channels, this is future work

test/javascripts/unit/models/ai-persona-test.js

@@ -53,6 +53,7 @@ module("Discourse AI | Unit | Model | ai-persona", function () {
       rag_chunk_overlap_tokens: 10,
       rag_conversation_chunks: 10,
       question_consolidator_llm: "Question Consolidator LLM",
+      allow_chat: false,
     };
 
     const aiPersona = AiPersona.create({ ...properties });
@@ -92,6 +93,7 @@ module("Discourse AI | Unit | Model | ai-persona", function () {
       rag_chunk_overlap_tokens: 10,
       rag_conversation_chunks: 10,
       question_consolidator_llm: "Question Consolidator LLM",
+      allow_chat: false,
     };
 
     const aiPersona = AiPersona.create({ ...properties });