feat(observability): 引入错误链路 error_flow 元数据并区分上游/客户端错误

- 网关在本地 failover 时构建 error_flow 元数据（分类/决策/传播策略），写入 report_context
- scheduler-core 解析并透传 error_flow 至候选 extra_data
- admin usage 详情拆分 request/upstream/client/failure_summary 错误域，敏感上游错误标记为 suppressed
- 前端 RequestDetailDrawer 拆出"返回客户端"与"上游响应"双错误卡片
- HorizontalRequestTimeline 节点详情展示真实请求错误及 error_flow 标签

apps/aether-gateway/src/orchestration/classifier.rs

@@ -145,6 +145,23 @@ pub(crate) enum LocalFailoverClassification {
     RetryUpstreamFailure,
 }
 
+impl LocalFailoverClassification {
+    pub(crate) const fn as_str(self) -> &'static str {
+        match self {
+            Self::UseDefault => "use_default",
+            Self::StopStatusCode => "stop_status_code",
+            Self::StopErrorPattern => "stop_error_pattern",
+            Self::StopSemanticClientError => "stop_semantic_client_error",
+            Self::RetrySuccessPattern => "retry_success_pattern",
+            Self::RetrySemanticCompatibilityError => "retry_semantic_compatibility_error",
+            Self::RetrySemanticRateLimit => "retry_semantic_rate_limit",
+            Self::RetrySemanticThinkingError => "retry_semantic_thinking_error",
+            Self::RetryStatusCode => "retry_status_code",
+            Self::RetryUpstreamFailure => "retry_upstream_failure",
+        }
+    }
+}
+
 pub(crate) fn classify_local_failover(
     policy: &LocalFailoverPolicy,
     input: LocalFailoverInput<'_>,

apps/aether-gateway/src/orchestration/mod.rs

@@ -1,4 +1,5 @@
 use aether_contracts::ExecutionPlan;
+use serde_json::{json, Value};
 
 use crate::AppState;
 
@@ -78,3 +79,43 @@ pub(crate) async fn resolve_local_failover_decision_for_attempt(
     .await
     .decision
 }
+
+pub(crate) fn build_local_error_flow_metadata(
+    status_code: u16,
+    response_text: Option<&str>,
+    analysis: LocalFailoverAnalysis,
+) -> Value {
+    let safe_to_expose = matches!(
+        analysis.classification,
+        LocalFailoverClassification::StopSemanticClientError
+            | LocalFailoverClassification::StopStatusCode
+            | LocalFailoverClassification::StopErrorPattern
+    );
+    let propagation = match analysis.decision {
+        LocalFailoverDecision::RetryNextCandidate => "suppressed",
+        LocalFailoverDecision::StopLocalFailover if safe_to_expose => "converted",
+        LocalFailoverDecision::StopLocalFailover => "suppressed",
+        LocalFailoverDecision::UseDefault if status_code >= 400 => "passthrough",
+        LocalFailoverDecision::UseDefault => "none",
+    };
+    json!({
+        "stage": "candidate",
+        "source": "upstream_response",
+        "status_code": status_code,
+        "classification": analysis.classification.as_str(),
+        "decision": analysis.decision.as_str(),
+        "retryable": matches!(analysis.decision, LocalFailoverDecision::RetryNextCandidate),
+        "safe_to_expose": safe_to_expose,
+        "propagation": propagation,
+        "message": local_failover_error_message(response_text),
+    })
+}
+
+pub(crate) fn with_error_flow_report_context(
+    report_context: Option<&Value>,
+    error_flow: Value,
+) -> Option<Value> {
+    let mut object = report_context?.as_object()?.clone();
+    object.insert("error_flow".to_string(), error_flow);
+    Some(Value::Object(object))
+}