Zed (and any other ACP client) never learned about a model switch that
happened from inside the agent loop — prewalk hand-offs, retry-fallback,
model cycling — because #pushConfigOptionUpdate was only ever wired to
the client-initiated setSessionConfigOption/setSessionMode RPCs and to
the thinking_level_changed lifetime event. The model itself did switch
correctly (subsequent requests used the new model), but the client's
model picker/status bar kept showing the session's starting model.
#handleLifetimeEvent now also reacts to the model_changed event added
in the previous commit and re-pushes config_option_update. Extend the
existing thinking-only subscription dedupe in setSessionConfigOption to
cover the model config id too, so a client-initiated model change still
produces exactly one notification once the lifetime subscription is
installed.
Regression tests mirror the existing thinking-level coverage:
- 'pushes config_option_update when the model changes internally'
- 'emits a single config_option_update per setSessionConfigOption(model) call'
Verified: bun test test/acp-agent.test.ts (57/57), plus
agent-session-prewalk.test.ts, agent-session-retry-fallback.test.ts,
retry-fallback.test.ts, model-resolver.test.ts, and the other acp-*.test.ts
files all still pass; tsgo --noEmit and biome check clean.
(cherry picked from commit f5c5081088e8cbac2650a9de89049731de1b2777)
