fix(tui): route prompts by next-step capability

packages/ui/tui/tests/harness.ts

@@ -36,6 +36,8 @@ interface FakeAgent extends Agent {
 
 export interface TuiHarnessOptions {
   status?: AgentStatus
+  /** Override the fake agent's next-step capability independently of status. */
+  acceptsNextStep?: boolean
   config?: Config
   /** Leave the session event log empty instead of seeding one turn and step. */
   omitInitialLifecycle?: boolean
@@ -191,6 +193,9 @@ export async function createTuiTestHarness<TerminalType extends Terminal, Exit e
     options: options.agentOptions ?? { provider: 'deepseek', model: 'deepseek-v4-flash' },
     session,
     status: options.status ?? 'idle',
+    get acceptsNextStep() {
+      return options.acceptsNextStep ?? this.status === 'running'
+    },
     ctx,
     sent,
     sentOptions,