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.

cmd/testdata/aiagent/init.go

@@ -87,6 +87,7 @@ func buildSeedItems(aiConfigID int64, knowledgeIDs []int64, defaultTeamIDs strin
 			ReplyTimeoutSeconds: 180,
 			TeamIDs:             defaultTeamIDs,
 			HandoffMode:         enums.AIAgentHandoffModeWaitPool,
+			FallbackMode:        enums.AIAgentFallbackModeSuggestRetry,
 			FallbackMessage:     "我暂时没有找到足够准确的信息。你可以补充具体的问题，我再继续帮你查。",
 			KnowledgeIDs:        utils.JoinInt64s(knowledgeIDs),
 			SkillIDs:            defaultSkillIDs,