Address PR feedback

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/OpenApiStreamReader.cs

@@ -3,8 +3,6 @@
 
 using System;
 using System.IO;
-using System.Security.Cryptography;
-using System.Text;
 using System.Threading.Tasks;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Models;
@@ -44,7 +42,6 @@ public OpenApiDocument Read(Stream input, out OpenApiDiagnostic diagnostic)
         {
             var reader = new StreamReader(input);
             var result = new OpenApiTextReaderReader(_settings).Read(reader, out diagnostic);
-
             if (!_settings.LeaveStreamOpen)
             {
                 reader.Dispose();

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -390,17 +390,16 @@ public IOpenApiReferenceable ResolveReference(OpenApiReference reference)
         /// <returns>The hash value.</returns>
         public static string GenerateHashValue(OpenApiDocument doc)
         {
-            HashAlgorithm sha = SHA512.Create();
-            using var memoryStream = new MemoryStream();
-
-            using var cryptoStream = new CryptoStream(memoryStream, sha, CryptoStreamMode.Write);
+            using HashAlgorithm sha = SHA512.Create();
+            using var cryptoStream = new CryptoStream(Stream.Null, sha, CryptoStreamMode.Write);
             using var streamWriter = new StreamWriter(cryptoStream);
 
             var openApiJsonWriter = new OpenApiJsonWriter(streamWriter, new OpenApiJsonWriterSettings { Terse = true });
             doc.SerializeAsV3(openApiJsonWriter);
             openApiJsonWriter.Flush();
 
-            var hash = memoryStream.ToArray();
+            cryptoStream.FlushFinalBlock();
+            var hash = sha.Hash;
 
             return ConvertByteArrayToString(hash);
         }