fix(chat): 重复追问被拦截时再多问一次模型

防重复问句会把整段回复削成「这一段我记住了。」只剩一句套话时，用带纠偏说明的 system 再调一次 LLM，尽量避免用户只看到干巴巴_ack。仍只重试一次，并打日志与 meta 标记 duplicate_question_guard_llm_retry。

api/app/agents/chat/interview_state_hints.py

@@ -5,6 +5,9 @@ from __future__ import annotations
 import re
 from collections.abc import Iterable
 
+# 与 `apply_duplicate_question_guard` 中整段替换句一致；用于判定是否需触发二次生成。
+DUPLICATE_QUESTION_GUARD_FALLBACK_ZH = "这一段我记住了。"
+
 from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
 
 from app.agents.stage_constants import STAGE_DISPLAY_ZH, STAGE_SLOT_KEYS
@@ -316,7 +319,7 @@ def apply_duplicate_question_guard(
         if repeated:
             sentences = [s.strip() for s in _SENTENCE_SPLIT_RE.split(text) if s.strip()]
             kept = [s for s in sentences if "？" not in s and "?" not in s]
-            replacement = kept[0] if kept else "这一段我记住了。"
+            replacement = kept[0] if kept else DUPLICATE_QUESTION_GUARD_FALLBACK_ZH
             if not replacement.endswith(("。", "！", "…")):
                 replacement += "。"
             cleaned.append(replacement)
@@ -324,10 +327,16 @@ def apply_duplicate_question_guard(
         else:
             cleaned.append(text)
     if not cleaned:
-        cleaned = ["这一段我记住了。"]
+        cleaned = [DUPLICATE_QUESTION_GUARD_FALLBACK_ZH]
     return cleaned, touched
 
 
+def segments_are_only_duplicate_guard_fallback(segments: Iterable[str]) -> bool:
+    """是否为「仅兜底_ack、无实质承接」——适合再打一枪模型。"""
+    parts = [str(s or "").strip() for s in segments if str(s or "").strip()]
+    return len(parts) == 1 and parts[0] == DUPLICATE_QUESTION_GUARD_FALLBACK_ZH
+
+
 def stage_slot_hint_lines(stage: str) -> list[str]:
     keys = STAGE_SLOT_KEYS.get(stage, ())
     stage_zh = STAGE_DISPLAY_ZH.get(stage, stage)