Add synthetic display message hook for middleware

[brainlid]

Problem

Middleware sometimes needs to record user-facing transcript entries that don't correspond to a LangChain.Message from the LLM. Two concrete cases:

• An ask_user middleware needs the user's answer to appear in the conversation as a user message, even though it never went through the model as a user message (it's a tool response).
• A cancellation flow needs a system-style notification like "User cancelled" to be visible in the transcript on reload.

There was no supported path for middleware to persist these entries through the same display-message pipeline that LLM-generated messages use, and therefore no way to get them broadcast to LiveViews via the existing :display_message_saved event.

Solution

Add a public AgentServer API that middleware can call to persist a synthetic display message, plus an optional callback on the DisplayMessagePersistence behaviour that integrators implement to actually write it.

• Sagents.AgentServer.save_synthetic_message_from/2 casts to the AgentServer, which routes the request to the configured persistence module and broadcasts the saved record as {:display_message_saved, msg} — the same event LiveViews already subscribe to for LLM messages.
• The new save_synthetic_message/3 callback is declared optional on the behaviour, so existing implementations don't break. AgentServer checks function_exported?/3 before calling and logs a warning if missing.
• Persistence failures ({:error, _} or raised exceptions) are logged but never propagate — synthetic messages are a transcript convenience and must not destabilise the agent.
• Generator template (priv/templates/display_message_persistence.ex.eex) gets a default implementation that delegates to the integrator's Conversations context, so newly generated projects pick this up automatically.

Changes

• lib/sagents/agent_server.ex — Added save_synthetic_message_from/2 public API, :save_synthetic_message cast handler, and the maybe_save_synthetic_and_broadcast/2 helper that gates on persistence + conversation_id, checks the optional callback, and rescues errors.
• lib/sagents/display_message_persistence.ex — Added synthetic_message_attrs typedoc and save_synthetic_message/3 optional callback with full @doc covering parameters, return shape, and the broadcast contract.
• priv/templates/display_message_persistence.ex.eex — Default generated implementation delegating to Conversations.append_display_message/3, with a {:error, :no_conversation} guard for missing conversation_id.
• test/sagents/agent_server_synthetic_message_test.exs — New test module covering the happy path, both no-op cases (missing conversation_id, missing persistence module), {:error, _} returns, and rescued exceptions.
• test/support/test_display_message_persistence.ex — Extended TestDisplayMessagePersistenceForwarding with save_synthetic_message/3 plus set_synthetic_response/1 and clear_synthetic_response/0 helpers driven via :persistent_term, so a single test module can simulate ok/error/raise outcomes.

Testing

mix test test/sagents/agent_server_synthetic_message_test.exs covers all five scenarios listed above. No live API calls — the forwarder persistence module substitutes for a real implementation and asserts the AgentServer's behaviour around it.