Fix DecoderNLS.Convert to out the correct value for 'completed' (#27210)

This is a complementary fix to https://github.com/dotnet/coreclr/issues/23020

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

src/Common/src/CoreLib/System/Text/DecoderNLS.cs

@@ -207,11 +207,13 @@ public override unsafe int GetCharCount(byte* bytes, int count, bool flush)
             charsUsed = _encoding.GetChars(bytes, byteCount, chars, charCount, this);
             bytesUsed = _bytesUsed;
 
-            // Its completed if they've used what they wanted AND if they didn't want flush or if we are flushed
-            completed = (bytesUsed == byteCount) && (!flush || !this.HasState) &&
-                               (_fallbackBuffer == null || _fallbackBuffer.Remaining == 0);
+            // Per MSDN, "The completed output parameter indicates whether all the data in the input
+            // buffer was converted and stored in the output buffer." That means we've successfully
+            // consumed all the input _and_ there's no pending state or fallback data remaining to be output.
 
-            // Our data thingy are now full, we can return
+            completed = (bytesUsed == byteCount)
+                && !this.HasState
+                && (_fallbackBuffer is null || _fallbackBuffer.Remaining == 0);
         }
 
         public bool MustFlush => _mustFlush;

src/Common/src/CoreLib/System/Text/EncoderNLS.cs

@@ -201,8 +201,6 @@ public override unsafe int GetBytes(char* chars, int charCount, byte* bytes, int
             completed = (charsUsed == charCount)
                 && !this.HasState
                 && (_fallbackBuffer is null || _fallbackBuffer.Remaining == 0);
-
-            // Our data thingys are now full, we can return
         }
 
         public Encoding Encoding