feat(transport): 暴露同格式 provider 的细分失败原因并在时间线展示

- claude_code/kiro/vertex policy 新增 transport_unsupported_reason_with_network 变体，返回具体失败码
- planner candidate_metadata 在 request_pair 写入同格式 provider 的细分 transport 原因
- 前端请求时间线优先展示 transport_diagnostics 的细分原因而非通用 transport_unsupported

frontend/src/features/usage/components/HorizontalRequestTimeline.vue

@@ -435,11 +435,11 @@
 
                 <!-- 跳过原因 -->
                 <div
-                  v-if="currentAttempt.skip_reason"
+                  v-if="currentAttemptSkipReasonDisplay"
                   class="skip-reason"
                 >
                   <span class="reason-label">跳过原因</span>
-                  <span class="reason-value">{{ currentAttempt.skip_reason }}</span>
+                  <span class="reason-value">{{ currentAttemptSkipReasonDisplay }}</span>
                 </div>
 
                 <!-- 错误信息 -->
@@ -1370,6 +1370,23 @@ const currentAttemptConversionInfo = computed<{
   return { summary, hint }
 })
 
+const currentAttemptSkipReasonDisplay = computed(() => {
+  const attempt = currentAttempt.value
+  if (!attempt?.skip_reason) return ''
+
+  if (attempt.skip_reason !== 'transport_unsupported') {
+    return attempt.skip_reason
+  }
+
+  const transportDiagnostics = resolveTransportDiagnostics(attempt)
+  const requestPair = extractObject(transportDiagnostics?.request_pair)
+  const detailedReason = typeof requestPair?.transport_unsupported_reason === 'string'
+    ? requestPair.transport_unsupported_reason.trim()
+    : ''
+
+  return detailedReason || attempt.skip_reason
+})
+
 // 计算当前尝试启用的能力标签（请求需要的能力）
 const activeCapabilities = computed(() => {
   if (!currentAttempt.value?.required_capabilities) return []