fix(usage): reject transient transport failures so retry and keep-last-good work

Usage/quota queries frequently showed spurious "query failed" states that
manual refresh could not clear (#3820). Root cause: all transport-level
failures were folded into Ok(success:false), so react-query's retry never
fired and the failure body poisoned the cache as regular data.

Backend:
- balance/coding_plan/subscription services now return Err for send
  failures and body-read failures (read body via bytes() before
  serde_json::from_slice; reqwest's json() wraps read errors as Decode,
  making error-kind checks on it dead code). Auth/4xx/parse errors stay
  Ok(success:false) and surface immediately.
- Script path maps transient AppError keys (request_failed /
  read_response_failed) to Err; Volcengine adds a Transient call variant.
- Command layer skips snapshot persistence, usage-cache-updated emit and
  tray refresh on Err so the cache bridge cannot overwrite retained data.
- Expired-credential retry propagates transient errors instead of
  rewriting them as "OAuth token has expired".

Frontend:
- resolveDisplayUsage generalized to subscription quotas; 5th param is now
  an options object with a `rejected` flag: stale success data retained by
  react-query across rejections is re-anchored to dataUpdatedAt and expires
  through the same 10-minute keep-last-good window instead of being shown
  indefinitely (a total outage used to mask longer than a single 5xx).
- useSubscriptionQuota / useCodexOauthQuota gain keep-last-good with
  per-scope snapshot reset; rejected queries with no displayable value
  synthesize a failure placeholder carrying the real error message so
  footers keep rendering a retry entry point.
- HTTP 429 is classified transient (retry-later) alongside 5xx.
- UsageScriptModal surfaces string rejections via extractErrorMessage.

Tests: 6 backend behavior tests drive real HTTP against a local listener
to pin the Err/Ok channel semantics; frontend suite extends
keepLastGoodUsage coverage (405 passing).

src/components/UsageScriptModal.tsx

@@ -8,6 +8,7 @@ import { usageApi, settingsApi, type AppId } from "@/lib/api";
 import { copilotGetUsage, copilotGetUsageForAccount } from "@/lib/api/copilot";
 import { useSettingsQuery } from "@/lib/query";
 import { resolveManagedAccountId } from "@/lib/authBinding";
+import { extractErrorMessage } from "@/utils/errorUtils";
 import { useDarkMode } from "@/hooks/useDarkMode";
 import {
   extractCodexBaseUrl,
@@ -663,8 +664,10 @@ const UsageScriptModal: React.FC<UsageScriptModalProps> = ({
         );
       }
     } catch (error: any) {
+      // 后端命令 Err(String) 时 invoke 以裸字符串 reject（如瞬时网络失败），
+      // 直接读 .message 会得到 undefined。
       toast.error(
-        `${t("usageScript.testFailed")}: ${error?.message || t("common.unknown")}`,
+        `${t("usageScript.testFailed")}: ${extractErrorMessage(error) || t("common.unknown")}`,
         {
           duration: 5000,
         },