Add some assertions to Stream.Read in System.Text.Encoding.CodePages (#69414)

Add some assertions to Stream.Read in System.Text.Encoding.CodePages (#69414)

* Add some assertions to Stream.Read in System.Text.Encoding.CodePages

BaseCodePageEncoding is relying on Stream.Read to read a full buffer of data in a single read. This is safe to assume because the only stream ever used is an UnmanagedMemoryStream, which will just copy the data to the buffer.

Adding some asserts to the code to make this assumption explicit.