Add gpt-5.3 support in TiktokenTokenizer

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs

@@ -1044,6 +1044,7 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 ( "o4-mini-", ModelEncoding.O200kBase ),  // e.g. o4-mini
 
                                                                 // chat
+                                                                ( "gpt-5.3-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.2-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.1-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5-", ModelEncoding.O200kBase ),
@@ -1073,6 +1074,7 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 { "o4-mini", ModelEncoding.O200kBase },
 
                                                                 // chat
+                                                                { "gpt-5.3", ModelEncoding.O200kBase },
                                                                 { "gpt-5.2", ModelEncoding.O200kBase },
                                                                 { "gpt-5.1", ModelEncoding.O200kBase },
                                                                 { "gpt-5", ModelEncoding.O200kBase },

test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs

@@ -38,6 +38,7 @@ public class TiktokenTests
         public static Tokenizer GPT5 { get; } = TiktokenTokenizer.CreateForModel("gpt-5");
         public static Tokenizer GPT5_1 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.1");
         public static Tokenizer GPT5_2 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.2");
+        public static Tokenizer GPT5_3 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.3");
         public static Tokenizer Phi4 { get; } = TiktokenTokenizer.CreateForModel("phi-4");
         public static TiktokenTokenizer GptOss { get; } = TiktokenTokenizer.CreateForModel("gpt-oss-20b");
 
@@ -421,6 +422,8 @@ public void TestEncodeR50kBase()
         [InlineData("gpt-5.1-mini")]
         [InlineData("gpt-5.2")]
         [InlineData("gpt-5.2-mini")]
+        [InlineData("gpt-5.3")]
+        [InlineData("gpt-5.3-mini")]
         [InlineData("chatgpt-4o-")]
         [InlineData("gpt-4")]
         [InlineData("gpt-4-")]
@@ -541,6 +544,7 @@ public void TestEncodingNamesNegativeCases()
         [InlineData("gpt-5")]
         [InlineData("gpt-5.1")]
         [InlineData("gpt-5.2")]
+        [InlineData("gpt-5.3")]
         [InlineData("o1")]
         [InlineData("o3")]
         [InlineData("o4-mini")]