CB TE cannot create new conversations — add "new" conversation_id support

[rmdevpro]

Problem

The CB TE has no way to create a new conversation via the OpenAI endpoint. If no conversation_id is passed, it always falls back to the single persistent Imperator conversation. The get_context response includes a conversation_id but the Imperator ignores it, and the chat response never returns it to the client.

Context

The pMAD template is being split into two base TE packages:

• base-pmad-te-ncb — PostgresSaver-backed, no CB dependency (default)
• base-pmad-te-cb — CB-backed, calls get_context/store_message

Both will support three modes via the conversation_id field on the OpenAI endpoint:

1. Pass an ID → resume that conversation
2. Pass nothing → resume the default Imperator thread
3. Pass "new" → create a new conversation, return its ID in the response

The nCB variant handles this natively via LangGraph PostgresSaver thread_ids. The CB variant needs to:

• When receiving "new", call get_context without a conversation_id so the CB creates one
• Capture the returned conversation_id from the get_context response (currently ignored at imperator_flow.py init_context_node)
• Return the conversation_id in the chat completions response (currently missing)

What needs to change in context-broker

1. The TE init_context_node must read and propagate the conversation_id from the get_context response
2. The chat response must include the conversation_id so clients can continue the conversation
3. Support the "new" sentinel value to trigger new conversation creation