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/ai_request.go

@@ -52,6 +52,7 @@ type CreateAIAgentRequest struct {
 	ReplyTimeoutSeconds int                             `json:"replyTimeoutSeconds"`
 	TeamIDs             []int64                         `json:"teamIds"`
 	HandoffMode         enums.AIAgentHandoffMode        `json:"handoffMode"`
+	FallbackMode        enums.AIAgentFallbackMode       `json:"fallbackMode"`
 	FallbackMessage     string                          `json:"fallbackMessage"`
 	KnowledgeIDs        []int64                         `json:"knowledgeIds"`
 	SkillIDs            []int64                         `json:"skillIds"`