fix: use correct System.CommandLine v2.0.5 API for CLI invocation

CommandLineConfiguration doesn't exist in System.CommandLine 2.0.5.
Use rootCommand.Parse(args).InvokeAsync() instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: HavenDV

src/cli/Tiktoken.Cli/Program.cs

@@ -387,8 +387,7 @@ await Console.Error.WriteLineAsync(
     return 0;
 });
 
-var config = new CommandLineConfiguration(rootCommand);
-return await config.InvokeAsync(args).ConfigureAwait(false);
+return await rootCommand.Parse(args).InvokeAsync().ConfigureAwait(false);
 
 static void HandleStdinMode(
     string input,