feat(tui): allow backgrounding synchronous subagents (#30488)

packages/opencode/test/server/httpapi-exercise/index.ts

@@ -568,6 +568,17 @@ const scenarios: Scenario[] = [
     .get("/experimental/session", "experimental.session.list")
     .at((ctx) => ({ path: "/experimental/session?roots=false&archived=false", headers: ctx.headers() }))
     .json(200, array),
+  http.protected
+    .post("/experimental/session/{sessionID}/background", "experimental.session.background")
+    .mutating()
+    .seeded((ctx) => ctx.session({ title: "Background route owner" }))
+    .at((ctx) => ({
+      path: route("/experimental/session/{sessionID}/background", { sessionID: ctx.state.id }),
+      headers: ctx.headers(),
+    }))
+    .json(200, (body) => {
+      check(body === false, "background route should be a no-op without running subagents")
+    }),
   http.protected.get("/experimental/resource", "experimental.resource.list").json(),
   http.protected
     .post("/sync/history", "sync.history.list")