fix(gateway): AUTO 范围冲突的提示要放在 checkQuota，否则是死代码

上一版把消息加在请求路径的范围检查上，实测才发现它永远走不到：
AUTO 请求先经过 checkQuota(ctx, "AUTO")（在链被读取之前），范围不含 AUTO
时它当场返回旧文案「model \"AUTO\" is not allowed for this key」——那句会
把运维引去查配额，而配额并不是原因。

改为在 checkQuota 内对 AUTO 分流，输出说明真实原因与修法。请求路径那处
换成注释说明为何不再重复。

判据 1 条，断言 checkQuota 函数体内必须同时出现 isAuto(model) 与新文案
——放在别处不算数。变异（删掉该分支）导致编译失败，被捕获。

生产实测（临时把某密钥范围改成不含 AUTO 再恢复）：
  HTTP 403  model_not_allowed
  model \"AUTO\" is not in this key's model scope, so it cannot use AUTO;
  add \"AUTO\" to the key's models or remove the scope restriction

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

internal/gateway/ui_key_auto_test.go

@@ -1,6 +1,7 @@
 package gateway
 
 import (
+	"os"
 	"strings"
 	"testing"
 )
@@ -362,3 +363,33 @@ func TestUIKeyAutoWarnsOnScopeConflict(t *testing.T) {
 		t.Fatalf("i18n key kAutoChainScopeWarn appears %d time(s), want zh+en", n)
 	}
 }
+
+// The scope conflict for AUTO must be reported by checkQuota, because that is
+// the check an AUTO request actually reaches first (it runs before the chain is
+// consulted). Putting the message on the later path made it dead code — the
+// request was rejected one check earlier with the generic "not allowed for this
+// key" wording, which sends an operator looking for a quota that isn't the
+// problem.
+func TestAutoScopeConflictMessageComesFromCheckQuota(t *testing.T) {
+	b, err := os.ReadFile("chat.go")
+	if err != nil {
+		t.Fatalf("read chat.go: %v", err)
+	}
+	src := string(b)
+	i := strings.Index(src, "func (g *Gateway) checkQuota(")
+	if i < 0 {
+		t.Fatal("checkQuota not found")
+	}
+	// Body = up to the next top-level func.
+	rest := src[i:]
+	if j := strings.Index(rest[1:], "\nfunc "); j > 0 {
+		rest = rest[:j+1]
+	}
+	if !strings.Contains(rest, "isAuto(model)") {
+		t.Fatal("checkQuota must special-case AUTO: a scope that omits AUTO is " +
+			"rejected there before anything else looks at the chain")
+	}
+	if !strings.Contains(rest, "not in this key's model scope") {
+		t.Fatal("checkQuota's AUTO branch must name the scope as the cause")
+	}
+}