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

[tmds]

When connected to a terminal reads happen line-by-line.
This was added in .NET 5 by #39192.

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.

Fixes#61865.

@dotnet/area-system-console ptal.

cc @dmbarbour

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Issue Details

---

When connected to a terminal reads happen line-by-line.
This was added in .NET 5 by #39192.

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.

Fixes #61865.

@dotnet/area-system-console ptal.

cc @dmbarbour

Author:	tmds
Assignees:	-
Labels:	area-System.Console
Milestone:	-

[adamsitnik]

@tmds thank you for your contribution! I've left some comments, PTAL

[tmds]

Adam, thank you for reviewing! I've addressed your feedback.

[adamsitnik]

@tmds thank you for addressing the feedback, LGTM.

[adamsitnik]

System.IO.FileSystem.Tests.DriveInfoWindowsTests.TestGetDrives test failure is a known issue (#62119) that I've just fixed (#62134). Other failures are #61643