Use Stream.ReadAtLeast when loading ZipArchives

[edwardneal]

Fixes#114026. Swaps references to Stream.Read out with Stream.ReadAtLeast, adds a test to prove the correct behaviour.

In every case except for ZipArchive, we use a pattern of calling Stream.ReadAtLeast(Span, Span.Length, throwOnEndOfStream: false) rather than Stream.ReadExactly(Span). The surrounding code checks the number of bytes read and returns false, and I didn't want to change that pattern.

The ZipArchive case takes place when reading the central directory, and it makes sure to always read enough bytes for the constant component of a ZipCentralDirectoryFileHeader.

/cc @carlossanlop

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[carlossanlop]

Thanks for this change, @edwardneal. I left some comments/questions for you to consider.

[carlossanlop]

LGTM modulo the debug question.

[carlossanlop]

/ba-g runtime-libraries-coreclr outerloop