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-tauri/src/commands/codex_oauth.rs

@@ -49,13 +49,14 @@ pub async fn get_codex_oauth_quota(
         }
     };
 
-    Ok(query_codex_quota(
+    // 瞬时传输失败以 Err 传播（前端 reject → retry + 保留上次成功值）。
+    query_codex_quota(
         &token,
         Some(&id),
         "codex_oauth",
         "Codex OAuth access token expired or rejected. Please re-login via cc-switch.",
     )
-    .await)
+    .await
 }
 
 /// 获取 Codex OAuth (ChatGPT Plus/Pro) 可用模型列表