fix(knowledge-guard): enhance fallback logic for knowledge retrieval and add tests

internal/ai/runtime/executor/knowledge_guard.go

@@ -15,12 +15,19 @@ type knowledgeGuardDecision struct {
 	Instructions  []*schema.Message
 }
 
+func buildKnowledgeUnavailableDecision(aiAgent models.AIAgent, knowledgeBaseIDs []int64) knowledgeGuardDecision {
+	if len(knowledgeBaseIDs) == 0 {
+		return knowledgeGuardDecision{}
+	}
+	return knowledgeGuardDecision{FallbackReply: resolveKnowledgeFallbackReply(aiAgent)}
+}
+
 func buildKnowledgeGuardDecision(aiAgent models.AIAgent, retrieveResult *retrievers.KnowledgeRetrieveResult) knowledgeGuardDecision {
 	if retrieveResult == nil || len(retrieveResult.KnowledgeBaseIDs) == 0 {
 		return knowledgeGuardDecision{}
 	}
 	fallbackReply := resolveKnowledgeFallbackReply(aiAgent)
-	if len(retrieveResult.Hits) == 0 {
+	if len(retrieveResult.Hits) == 0 || strings.TrimSpace(retrieveResult.ContextText) == "" {
 		return knowledgeGuardDecision{FallbackReply: fallbackReply}
 	}
 	instruction := buildKnowledgeRuntimeInstruction(retrieveResult.AnswerMode, fallbackReply)
@@ -50,7 +57,7 @@ func buildKnowledgeRuntimeInstruction(answerMode enums.KnowledgeAnswerMode, fall
 		fallbackReply = "当前知识库暂无明确信息。"
 	}
 	if answerMode == enums.KnowledgeAnswerModeAssist {
-		return "知识库回答约束：优先依据后续提供的知识片段回答，可以做轻度归纳，但不要编造片段中未提供的事实。若知识片段不足以直接支持答案，必须明确回复：" + fallbackReply
+		return "知识库回答约束：优先依据后续提供的知识片段回答，可以做轻度归纳，但不要编造片段中未提供的事实。回答中的具体事实、步骤、承诺必须能被知识片段直接支持；若知识片段不足以直接支持答案，必须明确回复：" + fallbackReply
 	}
-	return "知识库回答约束：本轮只能依据后续提供的知识片段回答，不得使用模型常识补充未提供的事实。若知识片段不足以支持回答，必须明确回复：" + fallbackReply
+	return "知识库回答约束：本轮只能依据后续提供的知识片段回答，不得使用模型常识补充未提供的事实，不得输出知识片段外的具体事实、步骤、承诺或建议。若知识片段不足以支持回答，必须明确回复：" + fallbackReply
 }