## Summary of the Pull Request
The "virtual bottom" marks the last line of the mutable viewport area, which is the part of the buffer that VT sequences can write to. This region should typically only move downwards, as new lines are added to the buffer, but there were a number of cases where it was incorrectly being moved up. This PR attempts to fix that.
## PR Checklist
* [x] Closes#9754
* [x] CLA signed.
* [x] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #9754
## Detailed Description of the Pull Request / Additional comments
When a call is made to `UpdateBottom`, we now clamp the value so it's at least as low as the current viewport bottom (i.e. if the viewport has moved down, we want the virtual bottom to move down too), but no lower than the bottom of the buffer (we don't want it to be out of range).
There is one special case where we do actually want the virtual bottom to move up - when the scrollback has been cleared with an `ED3` escape sequence. So in that case we needed a new `ConGetSet` API (`ResetBottom`) to reset the virtual bottom to the top of the buffer (essentially one less than the viewport height, since the virtual bottom points to the last line of the viewport).
## Validation Steps Performed
I had to reset the virtual bottom manually in some parts of the `ScreenBufferTests`, since some of the tests were relying on the virtual bottom being automatically reset when the viewport was reset, which is no longer the case.
I've also added a new test to verify that the virtual bottom doesn't move upwards if an update is triggered while the visible viewport is scrolled up. This essentially reproduces the test case from issue #9754, which I've also manually confirmed is fixed.
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