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/ui_key_auto_test.go

@@ -329,3 +329,36 @@ func TestUIKeyAutoNoticeDistinguishesSavedFromCopied(t *testing.T) {
 		}
 	}
 }
+
+// A key whose model scope excludes AUTO cannot use AUTO at all — the scope
+// filter runs before the chain — yet the editor happily lets an admin
+// configure a per-key chain. That combination is easy to create by accident
+// and produces a 403 with no visible cause, so the dialog must say so.
+//
+// We deliberately do NOT widen the scope automatically: silently granting a
+// model the operator did not ask for is worse than a loud warning.
+func TestUIKeyAutoWarnsOnScopeConflict(t *testing.T) {
+	src := uiSource(t)
+	if !strings.Contains(src, "function keyAutoScopeWarning(") {
+		t.Fatal("no scope-conflict check in the per-key editor")
+	}
+	body := readFuncBody(t, src, "keyAutoScopeWarning")
+	if !strings.Contains(body, "AUTO") {
+		t.Fatal("keyAutoScopeWarning must test whether the scope lists AUTO")
+	}
+	// It must not write anything back to the key: reporting only.
+	for _, forbidden := range []string{"PUT", "POST", "fetch(", "api("} {
+		if strings.Contains(body, forbidden) {
+			t.Fatalf("keyAutoScopeWarning calls %s — it must only report the "+
+				"conflict, never widen the scope itself", forbidden)
+		}
+	}
+	// Empty scope means unrestricted, so no warning is correct there.
+	if !strings.Contains(body, "if (!models.length)") {
+		t.Fatal("an unrestricted key (no models) must not be warned")
+	}
+	// Both languages.
+	if n := strings.Count(src, "kAutoChainScopeWarn"); n < 2 {
+		t.Fatalf("i18n key kAutoChainScopeWarn appears %d time(s), want zh+en", n)
+	}
+}