fix(usage): streaming 状态在首字节到达前显示为 pending

新增 resolveDisplayRequestStatus 函数，当记录状态为 streaming 但
first_byte_time_ms 为空时回退显示为 pending，避免误导用户。

frontend/src/features/usage/utils/__tests__/status.spec.ts

@@ -5,6 +5,7 @@ import {
   isUsageRecordSuccessful,
   mapRequestStatusToTimelineStatus,
   normalizeRequestStatus,
+  resolveDisplayRequestStatus,
   resolveTimelineFinalStatus,
 } from '../status'
 import type { UsageRecord } from '../../types'
@@ -64,6 +65,18 @@ describe('usage status helpers', () => {
     expect(mapRequestStatusToTimelineStatus('failed')).toBe('failed')
   })
 
+  it('shows streaming as pending until first byte is recorded', () => {
+    expect(resolveDisplayRequestStatus(buildUsageRecord({
+      status: 'streaming',
+      first_byte_time_ms: undefined,
+    }))).toBe('pending')
+
+    expect(resolveDisplayRequestStatus(buildUsageRecord({
+      status: 'streaming',
+      first_byte_time_ms: 320,
+    }))).toBe('streaming')
+  })
+
   it('treats explicit success status code as authoritative for the timeline', () => {
     expect(resolveTimelineFinalStatus({
       traceFinalStatus: 'success',

frontend/src/features/usage/utils/status.ts

@@ -76,6 +76,16 @@ export function normalizeRequestStatus(status: RequestStatusLike): RequestStatus
   }
 }
 
+export function resolveDisplayRequestStatus(
+  record: Pick<UsageRecord, 'status' | 'first_byte_time_ms'>
+): RequestStatus | undefined {
+  const status = normalizeRequestStatus(record.status)
+  if (status === 'streaming' && record.first_byte_time_ms == null) {
+    return 'pending'
+  }
+  return status
+}
+
 export function mapRequestStatusToTimelineStatus(
   status: RequestStatusLike
 ): TimelineFinalStatus | undefined {