Add knowledge retrieval configuration to Assistant model and related components

- Introduce new fields for knowledge retrieval configuration in AssistantConfig and Assistant models, including mode, top_n, and score_threshold.
- Implement KnowledgeRetrievalConfig schema with validation for top_n.
- Update backend services and routes to handle knowledge retrieval settings.
- Enhance frontend components to support knowledge retrieval configuration, including a new dialog for advanced settings.
- Add tests for knowledge retrieval configuration validation and description generation.

backend/db/models.py

@@ -164,6 +164,10 @@ class Assistant(Base):
     knowledge_base_id: Mapped[str | None] = mapped_column(
         String(40), ForeignKey("knowledge_bases.id", ondelete="RESTRICT"), nullable=True
     )
+    knowledge_retrieval_config: Mapped[dict] = mapped_column(
+        JSON,
+        default=lambda: {"mode": "automatic", "top_n": 5, "score_threshold": 0.0},
+    )
 
     # ---- 瘦类型专属字段(真列,稀疏:按 type 用其中几列) ----
     prompt: Mapped[str] = mapped_column(String(8192), default="")    # prompt / opencode