Console.Unix: fix OpenStandardInput Stream sometimes throwing for Reads. (#62153)

Console.Unix: fix OpenStandardInput Stream sometimes throwing for Reads. (#62153)

* Console.Unix: fix OpenStandardInput Stream sometimes throwing for Reads.

When connected to a terminal reads happen line-by-line.

StdInReader caches the line in a StringBuilder.

Instead of returning when the destination buffer is full, an attempt
was made to decode the next chunk from the StringBuilder (if there is
one). This causes the encoder to throw for encoding into an empty buffer.