Fix the buffering bug that blocks SDK (#51151)

* add a failing test that mimcs the SDK bug

* fix the buffering bug

src/libraries/System.IO.FileSystem/tests/FileStream/FileStreamConformanceTests.cs

@@ -161,6 +161,31 @@ public async Task LengthIsNotCachedAfterHandleHasBeenExposed(FileAccess fileAcce
             Assert.Equal(2, stream.Length);
             Assert.Equal(2, createdFromHandle.Length);
         }
+
+        [Fact]
+        public async Task WriteByteFlushesTheBufferWhenItBecomesFull()
+        {
+            string filePath;
+            List<byte> writtenBytes = new List<byte>();
+
+            using (FileStream stream = (FileStream)await CreateWriteOnlyStreamCore(Array.Empty<byte>()))
+            {
+                filePath = stream.Name;
+
+                stream.WriteByte(0);
+                writtenBytes.Add(0);
+
+                byte[] bytes = new byte[BufferSize - 1];
+                stream.Write(bytes.AsSpan());
+                writtenBytes.AddRange(bytes);
+
+                stream.WriteByte(1);
+                writtenBytes.Add(1);
+            }
+
+            byte[] allBytes = File.ReadAllBytes(filePath);
+            Assert.Equal(writtenBytes.ToArray(), allBytes);
+        }
     }
 
     public class UnbufferedSyncFileStreamStandaloneConformanceTests : FileStreamStandaloneConformanceTests

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

@@ -602,7 +602,7 @@ private void WriteByteSlow(byte value)
                 ClearReadBufferBeforeWrite();
                 EnsureBufferAllocated();
             }
-            else if (_writePos == _bufferSize - 1)
+            else
             {
                 FlushWrite();
             }