fix(model-fetch): support /models for Anthropic-compat subpath providers

Providers like DeepSeek, Kimi, Zhipu GLM and MiniMax expose the
Anthropic-compatible API on a subpath (e.g. /anthropic) while the
OpenAI-style /models endpoint lives at the API root. The previous
heuristic blindly appended /v1/models to the Base URL, so every such
provider returned 404 and the UI mislabeled it as "provider does not
support fetching models".

Backend now generates a candidate list and tries them in order:
preset override -> baseURL /v1/models -> stripped-subpath /v1/models ->
stripped-subpath /models. Non-404/405 responses (auth, network) stop
immediately so we never retry against hostile status codes. Known
compat suffixes are kept in a length-descending constant so the
longest match wins; response bodies are truncated to 512 chars to
avoid HTML 404 pages bloating the error string.

Preset type gains an optional modelsUrl (DeepSeek points at
https://api.deepseek.com/models). Frontend threads the override
through fetchModelsForConfig when the current Base URL still matches
the preset default. A new fetchModelsEndpointNotFound i18n key
replaces the misleading "not supported" toast for exhausted-candidate
and 404/405 cases (zh/en/ja).

src/i18n/locales/en.json

@@ -874,6 +874,7 @@
     "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",
+    "fetchModelsEndpointNotFound": "No reachable models endpoint found. Please check the Base URL or confirm whether the provider exposes this API.",
     "fetchModelsTimeout": "Request timed out, please check network connection"
   },
   "copilot": {

src/i18n/locales/ja.json

@@ -874,6 +874,7 @@
     "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",
+    "fetchModelsEndpointNotFound": "利用可能なモデル一覧エンドポイントが見つかりません。Base URL を確認するか、プロバイダーが該当 API を公開しているかご確認ください",
     "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": {

src/i18n/locales/zh.json

@@ -874,6 +874,7 @@
     "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",
+    "fetchModelsEndpointNotFound": "未找到可用的模型列表端点，请检查 Base URL 或确认供应商是否开放该接口",
     "fetchModelsTimeout": "请求超时，请检查网络连接"
   },
   "copilot": {