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/migrations/versions/20260712_0006_add_knowledge_retrieval_config.py

@@ -0,0 +1,32 @@
+"""add assistant knowledge retrieval config
+
+Revision ID: 20260712_0006
+Revises: 20260712_0005
+"""
+from collections.abc import Sequence
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "20260712_0006"
+down_revision: str | Sequence[str] | None = "20260712_0005"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "assistants",
+        sa.Column(
+            "knowledge_retrieval_config",
+            sa.JSON(),
+            server_default=sa.text(
+                "'{\"mode\": \"automatic\", \"top_n\": 5, \"score_threshold\": 0.0}'"
+            ),
+            nullable=False,
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("assistants", "knowledge_retrieval_config")