fix: Do not throw on read stream Flush

- Fixes unoplatform#5751 by making Flush a no-op for some of the Storage read-only streams

src/Uno.UI.RuntimeTests/Tests/Windows_Storage/Given_StorageFile_Native.base.cs

@@ -224,6 +224,72 @@ public async Task When_GetParent()
 			}
 		}
 
+		[TestMethod]
+		public async Task When_ReadStreamFlush()
+		{
+			var rootFolder = await GetRootFolderAsync();
+			var fileName = GetRandomTextFileName();
+			StorageFile? createdFile = null;
+			try
+			{
+				createdFile = await rootFolder.CreateFileAsync(fileName);
+				await FileIO.WriteTextAsync(createdFile, "Hello world!");
+
+				var buffer = new byte[1024];
+
+				using var stream = await createdFile.OpenStreamForReadAsync();
+
+				while (await stream.ReadAsync(buffer, 0, 1024) > 0)
+				{
+				}
+
+				try
+				{
+					stream.Flush();
+				}
+				catch (Exception ex)
+				{
+					Assert.Fail($"Read stream flush threw {ex}");
+				}
+			}
+			finally
+			{
+				await DeleteIfNotNullAsync(createdFile);
+				await CleanupRootFolderAsync();
+			}
+		}
+
+		[TestMethod]
+		public async Task When_InputStreamFlush()
+		{
+			var rootFolder = await GetRootFolderAsync();
+			var fileName = GetRandomTextFileName();
+			StorageFile? createdFile = null;
+			try
+			{
+				createdFile = await rootFolder.CreateFileAsync(fileName);
+				await FileIO.WriteTextAsync(createdFile, "Hello world!");
+
+				var buffer = new byte[1024];
+
+				using var stream = await createdFile.OpenReadAsync();
+
+				try
+				{
+					stream.FlushAsync();
+				}
+				catch (Exception ex)
+				{
+					Assert.Fail($"Input stream flush threw {ex}");
+				}
+			}
+			finally
+			{
+				await DeleteIfNotNullAsync(createdFile);
+				await CleanupRootFolderAsync();
+			}
+		}
+
 		private string GetRandomFolderName() => Guid.NewGuid().ToString();
 
 		private string GetRandomTextFileName() => Guid.NewGuid().ToString() + ".txt";

src/Uno.UI.RuntimeTests/Tests/Windows_Storage/Streams/Given_RandomAccessStreamReference.cs

@@ -24,6 +24,23 @@ public async Task When_FromUri()
 			Assert.AreEqual(_unoStaticTestFileContent, actual);
 		}
 
+		[TestMethod]
+		public async Task When_FlushReadOnly()
+		{
+			var sut = RandomAccessStreamReference.CreateFromUri(new Uri("https://nv-assets.azurewebsites.net/uno-unit-tests.txt"));
+			using var readStream = await sut.OpenReadAsync();
+
+			try
+			{
+				await readStream.FlushAsync();
+			}
+			catch (Exception)
+			{
+				// UWP throws NotImplementedException
+				// Uno throws InvalidOperationException with a description
+			}
+		}
+
 #if !NETFX_CORE
 		[TestMethod]
 		public async Task When_FromFile()
@@ -54,7 +71,7 @@ public async Task When_FromStream()
 			var tempContentBytes = Encoding.UTF8.GetBytes(tempContent);
 			temp.Write(tempContentBytes, 0, tempContentBytes.Length);
 			temp.Position = 0;
-			
+
 			var actual = await ReadToEnd(sut);
 
 			Assert.AreEqual(tempContent, actual);

src/Uno.UWP/Storage/Streams/Internal/NativeReadStream.wasm.cs

@@ -47,7 +47,7 @@ public RentedStream Rent()
 
 		public override void SetLength(long value) => throw new NotSupportedException("This stream is read-only");
 
-		public override void Flush() => throw new NotSupportedException("This stream is read-only");
+		public override void Flush() { }
 
 		public static async Task<NativeReadStream> CreateAsync(Guid fileId)
 		{

src/Uno.UWP/Storage/Streams/StreamOverInputStream.cs

@@ -65,11 +65,9 @@ public override Task WriteAsync(byte[] buffer, int offset, int count, Cancellati
 			=> throw new NotSupportedException("Cannot Write an input stream.");
 
 		/// <inheritdoc />
-		public override void Flush()
-			=> FlushAsync(CancellationToken.None).Wait();
+		public override void Flush() { }
 
 		/// <inheritdoc />
-		public override Task FlushAsync(CancellationToken cancellationToken)
-			=> throw new NotSupportedException("Cannot Flush an input stream.");
+		public override Task FlushAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 	}
 }