feat: add fallback mode for AI agents and update knowledge base handling

- Introduced AIAgentFallbackMode with options for NoAnswer and SuggestRetry.
- Updated AI agent edit component to include fallback mode selection.
- Modified API types and payloads to accommodate fallback mode.
- Removed deprecated KnowledgeFallbackMode and related logic from knowledge base.
- Adjusted knowledge retrieval and response handling to utilize new fallback logic.
- Updated tests to reflect changes in fallback mode handling for AI agents.

internal/pkg/dto/request/knowledge_request.go

@@ -14,7 +14,6 @@ type CreateKnowledgeBaseRequest struct {
 	ChunkMaxTokens        int     `json:"chunkMaxTokens"`
 	ChunkOverlapTokens    int     `json:"chunkOverlapTokens"`
 	AnswerMode            int     `json:"answerMode"`
-	FallbackMode          int     `json:"fallbackMode"`
 	Remark                string  `json:"remark"`
 }
 
@@ -71,7 +70,6 @@ type KnowledgeAnswerRequest struct {
 	SessionID        string  `json:"sessionId"`
 	ConversationID   int64   `json:"conversationId"`
 	AnswerMode       int     `json:"answerMode"`
-	FallbackMode     int     `json:"fallbackMode"`
 }
 
 type CreateKnowledgeFeedbackRequest struct {