Fixed cancelled ReadAsync/WriteAsync throwing the exception of the native operation instead of an OperationCanceledException

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Description

Fixes #2066.

Since 14.15.0 (when the AsyncStreamWrapper was introduced), cancelling ReadAsync/WriteAsync stream operations throws the exception of the native operation instead of an OperationCanceledException. For a cancelled JPEG write that is:

ImageMagick.MagickCorruptImageErrorException: Output file write error --- out of disk space? `' @ error/jpeg.c/JPEGErrorHandler/614

The cause: when the token is cancelled, the async pump loops set _exceptionThrown and the blocked native callback reports a failed read/write, which makes the native coder raise an error that propagates out of the action task. The existing conversion at the end of ReadAsync(Action, ...)/WriteAsync(Action, ...):

if (_exceptionThrown)
    cancellationToken.ThrowIfCancellationRequested();

is unreachable in that case because await Task.WhenAll(...) rethrows the action task's exception first.

This change wraps the Task.WhenAll in a try/catch when (_exceptionThrown) that calls ThrowIfCancellationRequested() (and rethrows the original exception when the failure was not caused by a cancellation, preserving the current behavior for stream exceptions).

The existing cancellation tests did not catch this because their test actions ignore the failed read/write result; the native coders throw in that situation. The new tests mirror the existing ones but throw from the action when the operation fails, plus end-to-end tests on MagickImage.ReadAsync(Stream)/WriteAsync(Stream) with a cancelled token.

Verified with the Q8 x64 net8.0 test suite on Windows: all tests pass with the fix; the four new tests fail without it (they reproduce the exact exceptions from #2066).

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