修复非超时 transport 抠图失败被记为不可重试

record_matting_external_api_failure 计算 retryable 时把 connect(transport)
参数硬编码为 false，导致 DNS / 连接重置 / body-read 中断这类 status_code=None、
timeout=false 的传输失败落在 statusClass=transport 却 retryable=false，与
"transport failures actionable" 语义冲突，误导告警 / 重试分析。

- status_code=None 恰好等价于 statusClass=transport，据此派生 has_transport_error
  传入 retryable 计算，无需改签名或调用点。
- 抽出可测的 build_matting_external_api_failure_draft。
- 补回归测试：非超时 transport → transport ∧ retryable=true；上游 4xx → retryable=false。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server-rs/crates/api-server/src/external_api_audit.rs

@@ -149,6 +149,7 @@ pub(crate) struct ExternalApiAuditContext {
 
 /// 抠图供应商（BgFilter / 阿里云通用抠图）调用失败的统一失败审计入口。
 /// 即使随后兜底成功，供应商故障也必须进入 OTLP + tracking_event，不能只 warn! 后静默。
+#[allow(clippy::too_many_arguments)]
 pub(crate) async fn record_matting_external_api_failure(
     state: &AppState,
     context: &ExternalApiAuditContext,
@@ -162,22 +163,52 @@ pub(crate) async fn record_matting_external_api_failure(
     error_message: String,
     raw_excerpt: Option<String>,
 ) {
-    let draft =
-        ExternalApiFailureDraft::new(provider, endpoint, operation, failure_stage, error_message)
-            .with_status_code(status_code)
-            .with_optional_status_class(Some(status_class(status_code)))
-            .with_timeout(timeout)
-            .with_retryable(is_retryable_external_api_failure(
-                status_code,
-                timeout,
-                false,
-            ))
-            .with_latency_ms(latency_ms)
-            .with_raw_excerpt(raw_excerpt)
-            .with_audit_context(context);
+    let draft = build_matting_external_api_failure_draft(
+        provider,
+        endpoint,
+        operation,
+        failure_stage,
+        status_code,
+        timeout,
+        latency_ms,
+        error_message,
+        raw_excerpt,
+        context,
+    );
     record_external_api_failure(state, draft).await;
 }
 
+#[allow(clippy::too_many_arguments)]
+fn build_matting_external_api_failure_draft(
+    provider: &'static str,
+    endpoint: String,
+    operation: &'static str,
+    failure_stage: &'static str,
+    status_code: Option<u16>,
+    timeout: bool,
+    latency_ms: Option<u64>,
+    error_message: String,
+    raw_excerpt: Option<String>,
+    context: &ExternalApiAuditContext,
+) -> ExternalApiFailureDraft {
+    // status_code=None ⟺ statusClass=transport（见 status_class）：DNS / 连接重置 / 读体中断 / 超时
+    // 这类传输层失败没有上游 HTTP 状态。它们必须与 "transport failures actionable" 语义一致，
+    // 记为 retryable=true，否则 statusClass=transport 却 retryable=false 会误导告警 / 重试分析。
+    let has_transport_error = status_code.is_none();
+    ExternalApiFailureDraft::new(provider, endpoint, operation, failure_stage, error_message)
+        .with_status_code(status_code)
+        .with_optional_status_class(Some(status_class(status_code)))
+        .with_timeout(timeout)
+        .with_retryable(is_retryable_external_api_failure(
+            status_code,
+            timeout,
+            has_transport_error,
+        ))
+        .with_latency_ms(latency_ms)
+        .with_raw_excerpt(raw_excerpt)
+        .with_audit_context(context)
+}
+
 pub(crate) fn matting_failure_audit_status_code(error: &AppError) -> Option<u16> {
     error
         .details()
@@ -534,6 +565,53 @@ mod tests {
         assert!(!is_retryable_external_api_failure(Some(400), false, false));
     }
 
+    #[test]
+    fn matting_failure_draft_marks_non_timeout_transport_retryable() {
+        // status_code=None、timeout=false 的非超时 transport 失败（DNS / 连接重置 / 读体中断）：
+        // statusClass 必须是 transport 且 retryable=true，否则与 "transport failures actionable" 冲突。
+        let draft = build_matting_external_api_failure_draft(
+            "bgfilter",
+            "https://bgfilter.example/remove-background".to_string(),
+            "editor-screen-background-removal",
+            "bgfilter_segment",
+            None,
+            false,
+            Some(67),
+            "请求 BgFilter 服务失败：dns error".to_string(),
+            Some("dns error".to_string()),
+            &ExternalApiAuditContext::default(),
+        );
+
+        assert_eq!(draft.status_code, None);
+        assert_eq!(draft.status_class, Some("transport"));
+        assert!(!draft.timeout);
+        assert!(
+            draft.retryable,
+            "非超时 transport 失败应当 retryable=true，与 statusClass=transport 保持一致"
+        );
+    }
+
+    #[test]
+    fn matting_failure_draft_keeps_client_error_non_retryable() {
+        // 真实上游 4xx（非 429 / 408）不是传输故障，仍应 retryable=false，不能被误判为可重试。
+        let draft = build_matting_external_api_failure_draft(
+            "aliyun-matting",
+            "imageseg.example".to_string(),
+            "editor-screen-background-removal",
+            "aliyun_segment",
+            Some(400),
+            false,
+            Some(12),
+            "通用抠图接口返回失败（HTTP 400，Code=InvalidImage）：bad image".to_string(),
+            None,
+            &ExternalApiAuditContext::default(),
+        );
+
+        assert_eq!(draft.status_code, Some(400));
+        assert_eq!(draft.status_class, Some("4xx"));
+        assert!(!draft.retryable);
+    }
+
     #[test]
     fn app_error_status_class_can_override_successful_upstream_status() {
         let draft = build_external_api_failure_tracking_draft(