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/provider.rs

@@ -381,32 +381,30 @@ pub async fn queryProviderUsage(
 ) -> Result<crate::provider::UsageResult, String> {
     let app_type = AppType::from_str(&app).map_err(|e| e.to_string())?;
     // inner 可能以两种形式失败：
-    //   1) 返回 Ok(UsageResult { success: false, .. }) —— 业务失败（401、脚本报错等）
-    //   2) 返回 Err(String) —— RPC/DB/Copilot fetch_usage 等 transport 层失败
-    // 两种都要把"失败"写进 UsageCache 并刷新托盘，让 format_script_summary 的
-    // success 守卫生效、suffix 自然消失，避免旧 success 快照长期滞留。
-    // 同时保持原始 Err 返回给前端 React Query 的 onError 回调，不吞错误。
+    //   1) 返回 Ok(UsageResult { success: false, .. }) —— 确定性失败（401、脚本
+    //      报错、未知供应商等）。写进 UsageCache 并刷新托盘，让
+    //      format_script_summary 的 success 守卫生效、suffix 自然消失。
+    //   2) 返回 Err(String) —— 瞬时传输失败（网络/超时）及 DB/Copilot fetch 等。
+    //      不写失败快照、不 emit：保留上一份托盘快照，与前端 react-query reject
+    //      保留上次 data 的语义一致；否则失败快照会经 useUsageCacheBridge 盲写
+    //      回 query 缓存，抹掉 reject 本该保留的旧值。
     let inner =
         query_provider_usage_inner(&state, &copilot_state, app_type.clone(), &providerId).await;
-    let snapshot = match &inner {
-        Ok(r) => r.clone(),
-        Err(err_msg) => crate::provider::UsageResult {
-            success: false,
-            data: None,
-            error: Some(err_msg.clone()),
-        },
-    };
-    let payload = serde_json::json!({
-        "kind": "script",
-        "appType": app_type.as_str(),
-        "providerId": &providerId,
-        "data": &snapshot,
-    });
-    if let Err(e) = app_handle.emit("usage-cache-updated", payload) {
-        log::error!("emit usage-cache-updated (script) 失败: {e}");
+    if let Ok(snapshot) = &inner {
+        let payload = serde_json::json!({
+            "kind": "script",
+            "appType": app_type.as_str(),
+            "providerId": &providerId,
+            "data": snapshot,
+        });
+        if let Err(e) = app_handle.emit("usage-cache-updated", payload) {
+            log::error!("emit usage-cache-updated (script) 失败: {e}");
+        }
+        state
+            .usage_cache
+            .put_script(app_type, providerId, snapshot.clone());
+        crate::tray::schedule_tray_refresh(&app_handle);
     }
-    state.usage_cache.put_script(app_type, providerId, snapshot);
-    crate::tray::schedule_tray_refresh(&app_handle);
     inner
 }