fix(session): ignore balanced no-step turns

packages/core/session/tests/session.spec.ts

@@ -6,11 +6,28 @@ import SessionStore, {
   Session,
   SessionEvent,
   SessionId,
+  findLastMessageTurnEnd,
   snapshotSessionEvent,
 } from '@deepseek-ai/dsh-session'
 import type { CreateSessionOptions, SessionEventType, SessionHeader, SessionSurface, TodoItem } from '@deepseek-ai/dsh-session'
 
 describe('Session', () => {
+  it('finds the latest closed turn that entered a model step', () => {
+    const session = new Session(SessionId('last-message-turn'))
+    session.append('turn/start', { turn: 1 })
+    session.append('turn/end', { turn: 1, reason: { kind: 'blocked' } })
+
+    expect(findLastMessageTurnEnd(session.events)).toBeUndefined()
+
+    session.append('turn/start', { turn: 2 })
+    session.append('step/start', { turn: 2, step: 1 })
+    session.append('step/end', { turn: 2, step: 1 })
+    session.append('turn/end', { turn: 2, reason: { kind: 'max-tokens' } })
+
+    expect(findLastMessageTurnEnd(session.events)?.data)
+      .toEqual({ turn: 2, reason: { kind: 'max-tokens' } })
+  })
+
   it('exposes one stable readonly surface view', () => {
     const session = new Session(SessionId('surface-view'))
     const surface = session.surface