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/response/knowledge_response.go

@@ -22,8 +22,6 @@ type KnowledgeBaseResponse struct {
 	ChunkOverlapTokens    int          `json:"chunkOverlapTokens"`
 	AnswerMode            int          `json:"answerMode"`
 	AnswerModeName        string       `json:"answerModeName"`
-	FallbackMode          int          `json:"fallbackMode"`
-	FallbackModeName      string       `json:"fallbackModeName"`
 	DocumentCount         int64        `json:"documentCount"`
 	FAQCount              int64        `json:"faqCount"`
 	Remark                string       `json:"remark"`