Correct horizontal coordinates in viewport overflow test

[j4james]

When resizing the buffer in the SetConsoleScreenBufferSize and
SetConsoleScreenBufferInfoEx APIs, we have tests in place to make sure
that the resize doesn't result in the viewport extending past the bottom
or right of the buffer (since that can eventually result in exceptions
being thrown). Unfortunately these tests were using the wrong X
coordinate, so they failed to detect an overflow along the horizontal
axis. This PR corrects that mistake.

PR #8309 was where the overflow detection was initially added.

The original code was using the Viewport::EndExclusive method to
determine the extent of the viewport, mistakenly thinking that it
returned the bottom right coordinates (it actually returns the left
coordinate). So I've now added a BottomRightExclusive method to the
Viewport class, that actually does return the coordinates we need, and
have updated the overflow tests to use that method instead.

Validation Steps Performed

I've manually confirmed that the test case is issue #8453 is no longer
throwing an exception.

Closes #8453

[zadjii-msft]

@msftbot make sure @miniksa signs off on this

[ghost]

Hello @zadjii-msft!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I'll only merge this pull request if it's approved by @miniksa

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".