feat: differentiate fetch models error messages by failure type

Distinguish between missing API key, missing endpoint, auth failure,
unsupported provider (404/405), and timeout errors instead of showing
a generic failure toast for all cases.

src/i18n/locales/en.json

@@ -795,7 +795,13 @@
     "fetchingModels": "Fetching...",
     "fetchModelsSuccess": "Found {{count}} models",
     "fetchModelsFailed": "Failed to fetch models",
-    "fetchModelsEmpty": "No models found"
+    "fetchModelsEmpty": "No models found",
+    "fetchModelsNeedApiKey": "Please fill in API Key first",
+    "fetchModelsNeedEndpoint": "Please fill in API endpoint first",
+    "fetchModelsNeedConfig": "Please fill in API endpoint and API Key first",
+    "fetchModelsAuthFailed": "API Key is invalid or lacks permission",
+    "fetchModelsNotSupported": "This provider does not support fetching model list",
+    "fetchModelsTimeout": "Request timed out, please check network connection"
   },
   "copilot": {
     "authSection": "GitHub Copilot Authentication",

src/i18n/locales/ja.json

@@ -795,7 +795,13 @@
     "fetchingModels": "取得中...",
     "fetchModelsSuccess": "{{count}}件のモデルを取得",
     "fetchModelsFailed": "モデル一覧の取得に失敗しました",
-    "fetchModelsEmpty": "モデルが見つかりません"
+    "fetchModelsEmpty": "モデルが見つかりません",
+    "fetchModelsNeedApiKey": "先に API Key を入力してください",
+    "fetchModelsNeedEndpoint": "先に API エンドポイントを入力してください",
+    "fetchModelsNeedConfig": "先に API エンドポイントと API Key を入力してください",
+    "fetchModelsAuthFailed": "API Key が無効か、権限がありません",
+    "fetchModelsNotSupported": "このプロバイダーはモデル一覧の取得に対応していません",
+    "fetchModelsTimeout": "リクエストがタイムアウトしました。ネットワーク接続を確認してください"
   },
   "copilot": {
     "authSection": "GitHub Copilot 認証",

src/i18n/locales/zh.json

@@ -795,7 +795,13 @@
     "fetchingModels": "正在获取...",
     "fetchModelsSuccess": "获取到 {{count}} 个模型",
     "fetchModelsFailed": "获取模型列表失败",
-    "fetchModelsEmpty": "未找到可用模型"
+    "fetchModelsEmpty": "未找到可用模型",
+    "fetchModelsNeedApiKey": "请先填写 API Key",
+    "fetchModelsNeedEndpoint": "请先填写 API 端点",
+    "fetchModelsNeedConfig": "请先填写 API 端点和 API Key",
+    "fetchModelsAuthFailed": "API Key 无效或无权限",
+    "fetchModelsNotSupported": "该供应商不支持获取模型列表",
+    "fetchModelsTimeout": "请求超时，请检查网络连接"
   },
   "copilot": {
     "authSection": "GitHub Copilot 认证",