Add test for Encoding class

src/test/php/com/openai/unittest/EncodingTest.class.php

@@ -0,0 +1,22 @@
+<?php namespace com\openai\unittest;
+
+use com\openai\Encoding;
+use test\{Assert, Test, Values};
+
+class EncodingTest {
+
+  #[Test]
+  public function named() {
+    Assert::equals(Encoding::$cl100k_base, Encoding::named('cl100k_base'));
+  }
+
+  #[Test, Values(['gpt-4o', 'gpt-4o-2024-05-13', 'o1-preview', 'omni'])]
+  public function for_omni_and_o1($model) {
+    Assert::equals(Encoding::$o200k_base, Encoding::for($model));
+  }
+
+  #[Test, Values(['gpt-4', 'gpt-4-0314', 'gpt-3.5-turbo-0301', 'gpt-35'])]
+  public function for_gpt35_and_40($model) {
+    Assert::equals(Encoding::$cl100k_base, Encoding::for($model));
+  }
+}