feat(chat): server-side interview turn plan (mode, anchor slot, snippet)

- Add plan_interview_turn: emotion_first / memoir_push / follow_user_only
- Inject hard directive block at top of guided system prompt
- Pass stage_switched_this_turn from ChatOrchestrator after stage detection
- Log interview_turn_plan for observability; add unit tests

api/tests/test_interview_turn_plan.py

@@ -0,0 +1,65 @@
+"""interview_turn_plan：轮次模式与主槽选择（服务端硬编排）。"""
+
+from app.agents.chat.interview_turn_plan import (
+    extract_anchor_snippet,
+    plan_interview_turn,
+    primary_empty_slot,
+)
+
+
+def test_primary_empty_slot_order():
+    assert primary_empty_slot("childhood", ["emotion", "place"]) == "place"
+    assert primary_empty_slot("childhood", ["emotion"]) == "emotion"
+
+
+def test_extract_anchor_snippet_prefers_memory():
+    mem = "摘录的一段记忆\n\n[场景氛围提示"
+    assert "摘录的一段记忆" in extract_anchor_snippet(
+        memory_evidence_text=mem, user_message="用户说很长一句" * 3
+    )
+
+
+def test_plan_memoir_push():
+    p = plan_interview_turn(
+        current_stage="childhood",
+        empty_slots=["place", "people"],
+        normalized_user_message="我小时候住在河边，夏天常去玩水。",
+        memory_evidence_text="",
+        stage_switched_this_turn=False,
+    )
+    assert p.mode == "memoir_push"
+    assert p.anchor_slot_key == "place"
+    assert p.anchor_snippet
+
+
+def test_plan_emotion_first():
+    p = plan_interview_turn(
+        current_stage="childhood",
+        empty_slots=["place"],
+        normalized_user_message="想起来还是很难受，忍不住想哭。",
+        memory_evidence_text="",
+        stage_switched_this_turn=False,
+    )
+    assert p.mode == "emotion_first"
+
+
+def test_plan_follow_on_stage_switch():
+    p = plan_interview_turn(
+        current_stage="education",
+        empty_slots=["school", "city"],
+        normalized_user_message="后来我去省城读中学了。",
+        memory_evidence_text="",
+        stage_switched_this_turn=True,
+    )
+    assert p.mode == "follow_user_only"
+
+
+def test_plan_follow_when_no_empty_slots():
+    p = plan_interview_turn(
+        current_stage="childhood",
+        empty_slots=[],
+        normalized_user_message="嗯。",
+        memory_evidence_text="",
+        stage_switched_this_turn=False,
+    )
+    assert p.mode == "follow_user_only"