Fix nullable annotations on MLKemAlgorithm and add ToString

Author: vcsjones

src/libraries/Common/src/System/Security/Cryptography/MLKemAlgorithm.cs

@@ -118,14 +118,17 @@ private MLKemAlgorithm(
         ///   <see langword="true" /> if the objects are considered equal; otherwise, <see langword="false" />.
         /// </returns>
         // This is a closed type, so all we need to compare are the names.
-        public bool Equals(MLKemAlgorithm? other) => other is not null && other.Name == Name;
+        public bool Equals([NotNullWhen(true)] MLKemAlgorithm? other) => other is not null && other.Name == Name;
 
         /// <inheritdoc />
-        public override bool Equals(object? obj) => obj is MLKemAlgorithm alg && alg.Name == Name;
+        public override bool Equals([NotNullWhen(true)] object? obj) => obj is MLKemAlgorithm alg && alg.Name == Name;
 
         /// <inheritdoc />
         public override int GetHashCode() => Name.GetHashCode();
 
+        /// <inheritdoc />
+        public override string ToString() => Name;
+
         /// <summary>
         ///   Determines whether two <see cref="MLKemAlgorithm" /> objects specify the same algorithm name.
         /// </summary>

src/libraries/Common/tests/System/Security/Cryptography/MLKemAlgorithmTests.cs

@@ -57,5 +57,12 @@ public static void Algorithms_Inquality()
             Assert.True(MLKemAlgorithm.MLKem768 != MLKemAlgorithm.MLKem1024, "MLKemAlgorithm.MLKem768 != MLKemAlgorithm.MLKem1024");
             Assert.True(MLKemAlgorithm.MLKem1024 != MLKemAlgorithm.MLKem512, "MLKemAlgorithm.MLKem1024 != MLKemAlgorithm.MLKem512");
         }
+
+        [Theory]
+        [MemberData(nameof(MLKemTestData.MLKemAlgorithms), MemberType = typeof(MLKemTestData))]
+        public static void Algorithms_ToString(MLKemAlgorithm algorithm)
+        {
+            Assert.Equal(algorithm.Name, algorithm.ToString());
+        }
     }
 }

src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs

@@ -1976,11 +1976,12 @@ internal MLKemAlgorithm() { }
         public string Name { get { throw null; } }
         public int PrivateSeedSizeInBytes { get { throw null; } }
         public int SharedSecretSizeInBytes { get { throw null; } }
-        public override bool Equals(object? obj) { throw null; }
-        public bool Equals(System.Security.Cryptography.MLKemAlgorithm? other) { throw null; }
+        public override bool Equals([System.Diagnostics.CodeAnalysis.NotNullWhenAttribute(true)] object? obj) { throw null; }
+        public bool Equals([System.Diagnostics.CodeAnalysis.NotNullWhenAttribute(true)] System.Security.Cryptography.MLKemAlgorithm? other) { throw null; }
         public override int GetHashCode() { throw null; }
         public static bool operator ==(System.Security.Cryptography.MLKemAlgorithm? left, System.Security.Cryptography.MLKemAlgorithm? right) { throw null; }
         public static bool operator !=(System.Security.Cryptography.MLKemAlgorithm? left, System.Security.Cryptography.MLKemAlgorithm? right) { throw null; }
+        public override string ToString() { throw null; }
     }
     [System.Diagnostics.CodeAnalysis.ExperimentalAttribute("SYSLIB5006", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
     public sealed partial class MLKemOpenSsl : System.Security.Cryptography.MLKem