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/Cargo.toml

@@ -16,7 +16,12 @@ to cortex (helexa's reverse-proxy / fleet gateway).
 # a painless migration to a dedicated GitHub repo in the future if the
 # project grows beyond helexa's needs. All deps are crates.io.
 [dependencies]
-agent-client-protocol = "0.12"
+# `unstable_session_model` flips on the SessionModelState type and the
+# session/set_model RPC the model-picker dropdown in Zed needs. The
+# feature is upstream-marked unstable; we accept that risk because the
+# model picker is core UX and the alternative (rolling our own
+# extension method) drifts further from spec each time it moves.
+agent-client-protocol = { version = "0.12", features = ["unstable_session_model"] }
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "io-util", "process", "signal"] }
 reqwest = { version = "0.12", features = ["json", "stream", "rustls-tls"], default-features = false }
 serde = { version = "1", features = ["derive"] }