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/controllers/dashboard/ai_agent_controller.go

@@ -124,6 +124,8 @@ func buildAIAgentResponse(item *models.AIAgent) response.AIAgentResponse {
 		ReplyTimeoutSeconds: item.ReplyTimeoutSeconds,
 		HandoffMode:         item.HandoffMode,
 		HandoffModeName:     enums.GetAIAgentHandoffModeLabel(item.HandoffMode),
+		FallbackMode:        item.FallbackMode,
+		FallbackModeName:    enums.GetAIAgentFallbackModeLabel(item.FallbackMode),
 		FallbackMessage:     item.FallbackMessage,
 		KnowledgeIDs:        utils.SplitInt64s(item.KnowledgeIDs),
 		SkillIDs:            utils.SplitInt64s(item.SkillIDs),