length: count runes, not bytes

MinLength/MaxLength measured len(value), so a string of 8 accented
characters (9+ bytes) failed MaxLength(8) despite the error message
promising "characters". Count with utf8.RuneCountInString so the check
matches the wording and the common-sense intent.

Still not grapheme-cluster aware (combining marks, emoji ZWJ sequences
count as several runes), which is out of scope for a stdlib-only helper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

length_test.go

@@ -20,7 +20,14 @@ func ExampleMinLength() {
 
 func TestMaxLength(t *testing.T) {
 	testCases := map[int]map[string]error{
-		8: {"abcd": nil, "abcdefgh": nil, "abcd efg": nil, "abcdefghi": ErrMustBeShorter.With(8)},
+		8: {
+			"abcd":      nil,
+			"abcdefgh":  nil,
+			"abcd efg":  nil,
+			"abcdéfgh":  nil, // 8 runes, 9 bytes
+			"abcdefghi": ErrMustBeShorter.With(8),
+			"abcdéfghi": ErrMustBeShorter.With(8), // 9 runes, 10 bytes
+		},
 	}
 
 	for setup, values := range testCases {
@@ -41,7 +48,14 @@ func TestMaxLength(t *testing.T) {
 
 func TestMinLength(t *testing.T) {
 	testCases := map[int]map[string]error{
-		8: {"abcd": ErrMustBeLonger.With(8), "abcdefgh": nil, "abcd efg": nil, "abcdefghi": nil},
+		8: {
+			"abcd":      ErrMustBeLonger.With(8),
+			"abcdéfg":   ErrMustBeLonger.With(8), // 7 runes, 8 bytes
+			"abcdefgh":  nil,
+			"abcdéfgh":  nil, // 8 runes, 9 bytes
+			"abcd efg":  nil,
+			"abcdefghi": nil,
+		},
 	}
 
 	for setup, values := range testCases {