fix(custom-model): wait for a freshly launched session to go idle before applying

Root cause of every 'Session is busy' apply failure reported from live
testing: a just-launched CLI reports itself 'busy' for its own startup
(boot spinner, workspace-trust check) well before runCustomModelEntry's
apply call could reach it, and the apply route's isBusy() guard correctly
cannot tell that apart from a real turn in progress — it exists precisely
to refuse restarting a session mid-turn, and a fresh boot looks exactly
like one from the outside. Confirmed live: replaying the identical apply
call by hand against the same session, once it had settled, succeeded
immediately.

Fixed by waiting on the session's own readiness signal before applying:
GET /api/sessions/:id/wait?until=idle&timeout=20000, one GET already built
for exactly this ('Agent wait primitives', CLAUDE.md) rather than inventing
a client-side poll loop. A timeout there is a normal 200 per that
endpoint's own contract, never an error, so a session still busy after 20s
just reaches the apply call anyway and gets the route's own honest error —
now visible, since the previous commit made error toasts sticky and
stopped discarding the real error text.

Tests: new case in custom-model-run-menu-ui.test.ts pins the ordering (the
wait call happens, and strictly before the apply call) and its exact query
string.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

docs/custom-model-endpoints.md

@@ -105,11 +105,21 @@ the point of asking is letting one launch deliberately differ from the
 saved default, not just confirming it. Whichever way the model was decided,
 the launch itself runs a single session on that harness exactly the way its
 own Run-menu entry would (same case creation, env overrides, everything),
-then immediately applies the endpoint and model to it via the route below.
-It is a one-off "try this endpoint" action, not a sticky mode: the plain
-Run button still means "this harness, native cloud" afterward. Entries are
-hidden entirely for a remote or Docker active case, since the apply route
-refuses both (see the next section).
+then **waits for the new session to go idle** (`GET .../wait?until=idle`,
+bounded at 20s — a normal 200 either way, never an error, per the wait
+endpoint's own contract) before applying the endpoint and model to it via
+the route below. That wait exists because a freshly launched CLI reports
+itself as `busy` for its own startup (a boot spinner, a workspace-trust
+check) well before the apply call would otherwise reach it, and the apply
+route correctly refuses to restart a session mid-turn — a fresh boot looks
+exactly like one from the outside. A session still busy after the wait
+reaches the apply call anyway and gets that route's own honest
+`SESSION_BUSY` error, now visible as a sticky toast with a close button
+rather than a generic message that vanished in three seconds. It is a
+one-off "try this endpoint" action, not a sticky mode: the plain Run button
+still means "this harness, native cloud" afterward. Entries are hidden
+entirely for a remote or Docker active case, since the apply route refuses
+both (see the next section).
 
 ## Applying a model to a session