feat(helexa-acp): model picker + session/set_model handler

Stage 4. Zed's model dropdown now lists every model from every
configured endpoint, and switching it routes the next prompt to a
new endpoint+model.

- Enable `unstable_session_model` on the agent-client-protocol dep
  so SessionModelState / SetSessionModelRequest / ModelInfo are
  available.
- Agent::new becomes async and calls Provider::list_models on every
  provider at startup; per-endpoint failures warn-and-skip instead
  of aborting the agent.
- With a single endpoint configured, model ids appear bare; with
  multiple endpoints every id carries the `endpoint:` prefix so the
  picker is unambiguous and parse_model_selector routes correctly.
- NewSessionResponse and LoadSessionResponse attach SessionModelState
  with the session's current model id + the aggregated catalogue.
- session/set_model: validates the requested model id against
  resolve_provider, mutates session.model_id, and persists so the
  on-disk transcript reflects the new model.

Three new aggregate_models tests cover the prefixing rule (bare vs
multi-endpoint) and warn-and-skip on a failing endpoint.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/helexa-acp/src/main.rs

@@ -142,6 +142,7 @@ async fn main() -> Result<()> {
     }
 
     let agent = Agent::new(&cfg, providers)
+        .await
         .map_err(|e| agent_client_protocol::util::internal_error(format!("agent: {e:#}")))?;
     agent.serve(Stdio::new()).await
 }