refactor(validation): inline annotation qualified name check (#5601)

Replace the single use of k8s.io/apimachinery/pkg/util/validation
.IsQualifiedName (annotation key validation in pkg/config/v1/validation)
with a local implementation in pkg/util/validation. Newer apimachinery
versions link klog only for IP/CIDR warning helpers, which grows frpc and
frps, and this was the only frp import of that package.

The local IsQualifiedName keeps the exact behavior and error messages of
apimachinery v0.28.8, including the qualified-name (label key) format and
the DNS-1123 subdomain prefix rules. Annotation keys are still lowercased
before validation, so the accepted set is unchanged.

Verified against apimachinery v0.28.8 on 228,220 inputs (exhaustive short
strings, boundary lengths and seeded random data, including lowercased
variants): every error message matches.

No dependency versions change; go.mod and go.sum are untouched.

pkg/util/validation/qualified_name_test.go

@@ -0,0 +1,102 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package validation
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsQualifiedNameValidity(t *testing.T) {
+	tests := []struct {
+		name  string
+		value string
+		valid bool
+	}{
+		{name: "single letter", value: "a", valid: true},
+		{name: "single digit", value: "0", valid: true},
+		{name: "uppercase name", value: "MyName", valid: true},
+		{name: "allowed punctuation", value: "a-._b", valid: true},
+		{name: "name at max length", value: strings.Repeat("a", 63), valid: true},
+		{name: "name over max length", value: strings.Repeat("a", 64), valid: false},
+		{name: "prefix and name", value: "example.com/MyName", valid: true},
+		{name: "prefix at max length", value: strings.Repeat("a", 253) + "/key", valid: true},
+		{name: "prefix over max length", value: strings.Repeat("a", 254) + "/key", valid: false},
+		{name: "name at max length with prefix", value: "example.com/" + strings.Repeat("a", 63), valid: true},
+		{name: "name over max length with prefix", value: "example.com/" + strings.Repeat("a", 64), valid: false},
+		{name: "empty", value: "", valid: false},
+		{name: "leading hyphen", value: "-a", valid: false},
+		{name: "trailing hyphen", value: "a-", valid: false},
+		{name: "leading underscore", value: "_a", valid: false},
+		{name: "trailing dot", value: "a.", valid: false},
+		{name: "only dot", value: ".", valid: false},
+		{name: "double dot in name", value: "a..b", valid: true},
+		{name: "space", value: "a b", valid: false},
+		{name: "tab", value: "a\tb", valid: false},
+		{name: "newline", value: "a\nb", valid: false},
+		{name: "nul byte", value: "a\x00b", valid: false},
+		{name: "non ascii name", value: "é", valid: false},
+		{name: "non ascii prefix", value: "é/key", valid: false},
+		{name: "empty prefix", value: "/key", valid: false},
+		{name: "empty name", value: "example.com/", valid: false},
+		{name: "two slashes", value: "a//b", valid: false},
+		{name: "three segments", value: "a/b/c", valid: false},
+		{name: "uppercase prefix", value: "Example.com/key", valid: false},
+		{name: "prefix with empty label", value: "example..com/key", valid: false},
+		{name: "prefix with leading hyphen", value: "-example.com/key", valid: false},
+		{name: "prefix with trailing dot", value: "example.com./key", valid: false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			errs := IsQualifiedName(tc.value)
+			if tc.valid {
+				require.Empty(t, errs)
+			} else {
+				require.NotEmpty(t, errs)
+			}
+		})
+	}
+}
+
+func TestIsQualifiedNameMessages(t *testing.T) {
+	t.Run("name too long", func(t *testing.T) {
+		errs := IsQualifiedName(strings.Repeat("a", 64))
+		require.Contains(t, errs, "name part must be no more than 63 characters")
+	})
+	t.Run("prefix too long", func(t *testing.T) {
+		errs := IsQualifiedName(strings.Repeat("a", 254) + "/key")
+		require.Contains(t, errs, "prefix part must be no more than 253 characters")
+	})
+	t.Run("too many segments", func(t *testing.T) {
+		errs := IsQualifiedName("a/b/c")
+		require.Len(t, errs, 1)
+		require.Contains(t, errs[0], "a qualified name must consist of alphanumeric characters")
+		require.Contains(t, errs[0], "with an optional DNS subdomain prefix and '/'")
+	})
+	t.Run("invalid character", func(t *testing.T) {
+		errs := IsQualifiedName("a b")
+		require.Contains(t, errs[0], "name part must consist of alphanumeric characters")
+	})
+	t.Run("empty prefix", func(t *testing.T) {
+		errs := IsQualifiedName("/key")
+		require.Equal(t, []string{"prefix part must be non-empty"}, errs)
+	})
+	t.Run("empty name", func(t *testing.T) {
+		errs := IsQualifiedName("example.com/")
+		require.Contains(t, errs[0], "name part must be non-empty")
+	})
+}