fix(server): make /compact slash-command test mock yield session.idle

The empty event-stream mock added in #597 races with the summarize-success
path: consumeEventStream sees the empty stream end immediately and emits a
"stream ended before terminal event" turn_failed which gets buffered before
session.run has assigned its local turnId. The buffered event replays after
startTurn returns, marks settled = true, and rejects completion — but
session.run skips await completion because settled is true, leaving the
rejection unhandled and tripping vitest.

Yield session.idle so the stream resolves the turn the same way real
OpenCode SSE does.

packages/server/src/server/agent/providers/opencode-agent.slash-command-timeout.test.ts

@@ -91,7 +91,12 @@ describe("OpenCodeAgentSession slash command timeout handling", () => {
       },
       event: {
         subscribe: vi.fn().mockResolvedValue({
-          stream: (async function* () {})(),
+          stream: (async function* () {
+            yield {
+              type: "session.idle",
+              properties: { sessionID: "session-1" },
+            };
+          })(),
         }),
       },
       command: {