Feat: support tree structured deep-research policy. (#12559)

### What problem does this PR solve?

#12558
### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/prompts/sufficiency_check.md

@@ -0,0 +1,24 @@
+You are a information retrieval evaluation expert. Please assess whether the currently retrieved content is sufficient to answer the user's question.
+
+User question:
+{{ question }}
+
+Retrieved content:
+{{ retrieved_docs }}
+
+Please determine whether these content are sufficient to answer the user's question.
+
+Output format (JSON):
+```json
+{
+    "is_sufficient": true/false,
+    "reasoning": "Your reasoning for the judgment",
+    "missing_information": ["Missing information 1", "Missing information 2"]
+}
+```
+
+Requirements:
+1. If the retrieved content contains key information needed to answer the query, judge as sufficient (true).
+2. If key information is missing, judge as insufficient (false), and list the missing information.
+3. The `reasoning` should be concise and clear.
+4. The `missing_information` should only be filled when insufficient, otherwise empty array.