feat: classify stream check errors with color-coded toasts

Distinguish between "provider rejects probe" (yellow warning) and
"genuinely broken" (red error) in health check results.

Backend: add AppError::HttpStatus variant to carry structured HTTP
status codes, populate http_status on error results, classify codes
into short labels (e.g. "Auth rejected (401)"), and truncate overly
long response bodies.

Frontend: route 401/403/400/429/5xx to toast.warning with localized
hints explaining the error may not indicate actual unusability; route
404/402/connection errors to toast.error. Add i18n keys for all three
locales (zh/en/ja).

Also deduplicate check_once by reusing build_stream_check_result.

src/components/providers/forms/ProviderAdvancedConfig.tsx

@@ -1,11 +1,6 @@
 import { useTranslation } from "react-i18next";
 import { useState, useEffect } from "react";
-import {
-  ChevronDown,
-  ChevronRight,
-  FlaskConical,
-  Coins,
-} from "lucide-react";
+import { ChevronDown, ChevronRight, FlaskConical, Coins } from "lucide-react";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { Switch } from "@/components/ui/switch";