fix(gateway): AUTO 被密钥模型范围拦下时报错误导 + 编辑器给出提示

一个密钥的模型范围不含 AUTO 时，它用不了 AUTO —— 范围过滤发生在链之前。
但这两条路径都不说真话：

1. 请求侧：AUTO 走的是通用分支，返回 model_not_found "model \"AUTO\" is
   not configured"，读起来像 AUTO 没配置。非 AUTO 模型早就有 model_not_allowed
   的专门提示，AUTO 漏了。补上，并说明修法（把 AUTO 加进该密钥的 models，
   或去掉范围限制）。

2. 编辑器侧：per-key AUTO 链编辑器可以正常配链、保存也成功，看起来一切正常，
   但该密钥的每个请求都会 403。管理员无从得知。现在弹窗顶部在检测到冲突时
   显示警告，并列出当前范围。

刻意不做的事：不自动把 AUTO 加进该密钥的模型范围。那等于悄悄授予运营
没要求的访问权，比一个显眼的警告更糟。

判据 1 条，除确认提示出现外还断言该函数体内没有 PUT/POST/fetch/api ——
它只报告，不得写回。变异（去掉 AUTO 判断）判红。

CDP 实测四种场景：范围含 AUTO → 无提示；范围不含 → 警告并列出范围；
空范围（不受限）→ 无提示；范围含 AUTO → 无提示。

Co-Authored-By: ModelRouter <noreply@modelrouter.dev>

internal/gateway/chat.go

@@ -452,6 +452,19 @@ func (g *Gateway) handleChat(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	}
+	// A key whose model scope does not include AUTO cannot use AUTO at all,
+	// even when it has its own AUTO chain configured. That combination is
+	// easy to set up by accident and produced a misleading error: the request
+	// fell through to the generic "model %q is not configured" below, which
+	// claims AUTO does not exist — it does, this key just may not use it. Name
+	// the actual reason so the admin can fix the scope.
+	if isAuto(model) {
+		if allow := g.allowedModels(r.Context()); allow != nil && !g.hasScopeModel(allow, model) {
+			writeError(w, http.StatusForbidden, "model_not_allowed",
+				fmt.Sprintf("model %q is not in this key's model scope, so it cannot use AUTO; add %q to the key's models or remove the scope restriction", model, model))
+			return
+		}
+	}
 	cands, effective := g.resolveCands(r.Context(), &req)
 	if len(cands) == 0 {
 		writeError(w, http.StatusNotFound, "model_not_found", fmt.Sprintf("model %q is not configured", model))