Disallow a single dot in IDN hostnames (#247)

Based on
json-schema-org/JSON-Schema-Test-Suite#759 it is
not allowed to have a single dot in the hostname (and IDN hostname). The
hostname validator already had the correct behavior but the IDN hostname
validator did not

json-schema-validator/src/commonMain/kotlin/io/github/optimumcode/json/schema/internal/formats/IdnHostnameFormatValidator.kt

@@ -49,7 +49,7 @@ internal object IdnHostnameFormatValidator : AbstractStringFormatValidator() {
       return FormatValidator.Invalid()
     }
     if (value.length == 1 && isLabelSeparator(value[0])) {
-      return FormatValidator.Valid()
+      return FormatValidator.Invalid()
     }
 
     // https://datatracker.ietf.org/doc/html/rfc5893#section-1.4
@@ -506,6 +506,11 @@ internal object IdnHostnameFormatValidator : AbstractStringFormatValidator() {
   private fun isACE(label: String): Boolean =
     label.length > Punycode.PREFIX_SIZE && label.startsWith(Punycode.PREFIX_STRING)
 
+  /**
+   * Returns `true` if the [c] is a dot
+   * according to [RFC3490 Section 3.1](https://datatracker.ietf.org/doc/html/rfc3490#section-3.1).
+   * Otherwise, returns `false`
+   */
   private fun isLabelSeparator(c: Char): Boolean = c == '.' || c == '\u3002' || c == '\uFF0E' || c == '\uFF61'
 
   private fun findDot(

json-schema-validator/src/commonTest/kotlin/io/github/optimumcode/json/schema/assertions/general/format/JsonSchemaIdnHostnameFormatValidationTest.kt

@@ -9,7 +9,6 @@ class JsonSchemaIdnHostnameFormatValidationTest : FunSpec() {
       format = "idn-hostname",
       validTestCases =
         listOf(
-          ".",
           "a",
           "hostname",
           // 63
@@ -30,6 +29,10 @@ class JsonSchemaIdnHostnameFormatValidationTest : FunSpec() {
       invalidTestCases =
         listOf(
           TestCase("", "empty value"),
+          TestCase(".", "single separator"),
+          TestCase("\u3002", "single separator U+3002"),
+          TestCase("\uFF0E", "single separator U+FF0E"),
+          TestCase("\uFF61", "single separator U+FF61"),
           TestCase("xn--80aakdqneodaeaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaai7g2bxc6qoj1f", "too long punycode"),
           TestCase("оооооооооооооооооооооооооооооооооооченьдлиннаястрока", "too long unicode"),
           // Not normalized \u4E3D. Example from https://unicode.org/Public/UNIDATA/NormalizationTest.txt