feat(stream-check): refresh default models and detect model-not-found errors (#2099)

* chore(stream-check): update default health check models to latest

Replaces deprecated gpt-5.1-codex@low with gpt-5.4@low and switches
the Gemini default from gemini-3-pro-preview to gemini-3-flash-preview
to pick the lightest variant of the latest series for fast, low-cost
health checks.

https://claude.ai/code/session_01NGWLchcTP76rJHjiP5Ehte

* feat(stream-check): detect model-not-found errors with dedicated toast

Health check previously classified failures purely by HTTP status code,
which meant deprecated/invalid models showed up as a generic "Not found
(404)" error pointing users to check the Base URL — misleading when the
URL is fine and only the test model is wrong (e.g. gpt-5.1-codex after
it was retired).

Backend: add detect_error_category() that inspects 4xx response bodies
for model-not-found indicators (model_not_found, does not exist,
invalid model, not_found_error, etc.) and returns a "modelNotFound"
category. Thread the resolved test model through build_stream_check_result
so the failed result carries it in model_used. Add StreamCheckResult
.error_category field (serde-skipped when None).

Frontend: useStreamCheck branches on errorCategory === "modelNotFound"
before the HTTP-status fallback and renders a toast.error with the model
name and a description pointing to Model Test Config. Add i18n keys
(modelNotFound / modelNotFoundHint) for zh/en/ja.

Tests: unit-test detect_error_category against real OpenAI/Anthropic
error shapes, 5xx false-positive avoidance, and plain 401 auth errors.

https://claude.ai/code/session_01NGWLchcTP76rJHjiP5Ehte

* fix(stream-check): add missing error_category field in fallback

The error_category field was added to StreamCheckResult in this branch
but the fallback constructor in stream_check_all_providers was not
updated, which broke cargo build.

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/hooks/useStreamCheck.ts

@@ -46,6 +46,22 @@ export function useStreamCheck(appId: AppId) {
 
           // 降级状态也重置熔断器，因为至少能通信
           resetCircuitBreaker.mutate({ providerId, appType: appId });
+        } else if (result.errorCategory === "modelNotFound") {
+          // 专门处理"模型不存在/已下架"：指向配置入口，比通用 404 文案更有指导性
+          toast.error(
+            t("streamCheck.modelNotFound", {
+              providerName: providerName,
+              model: result.modelUsed,
+              defaultValue: `${providerName} 测试模型 ${result.modelUsed} 不存在或已下架`,
+            }),
+            {
+              description: t("streamCheck.modelNotFoundHint", {
+                defaultValue: "",
+              }),
+              duration: 10000,
+              closeButton: true,
+            },
+          );
         } else {
           const httpStatus = result.httpStatus;
           const hintKey = httpStatus