fix(agent): Agent 运行 - 重复读取改为软收敛并保留可纠正的媒体失败原因

重复只读和同版本识图不再把运行标成失败，下一步暂时收起只读工具。媒体能力错误改为可纠正，准入失败保留安全原因。

backend/internal/app/cloud_agent_prompt_context.go

@@ -17,6 +17,7 @@ const (
 	cloudAgentContextEmptyOutput        cloudAgentRuntimeContextKind = "empty_output"
 	cloudAgentContextInvalidOutput      cloudAgentRuntimeContextKind = "invalid_output"
 	cloudAgentContextTruncatedArguments cloudAgentRuntimeContextKind = "truncated_tool_arguments"
+	cloudAgentContextReadLoop           cloudAgentRuntimeContextKind = "read_loop"
 )
 
 // Only serializable fact fields belong here. Behavioral instructions live in