Be able to switch the pane "focus" of my cursor programmatically

### Description of the new feature

I want to be able to run a command in one pane and then have my cursors focus be able to shift to the other frame. This is mainly for AI Shell where after a someone uses the command `/code post` to post code responses to their working shell that their cursor is also shifted to the left in order to help speed of running the commands suggested. Related issue: https://github.com/PowerShell/AIShell/issues/311

In the GIF below, I have to click on the left side to shift my focus and be able to execute the command inserted.


### Proposed technical implementation details

_No response_

The issue does not identify a source file, test, or entry point; begin by locating the existing pane-focus and command-handling paths in the Windows Terminal codebase. Done means exposing a programmatic way for a command such as AI Shell's `/code post` flow to move cursor focus to the other pane, with behavior covered by appropriate tests.

Written by the indexing model from the issue text.