Support interaction with terminal windows embedded in Gemini CLI

[jacob314]

This will address issues such as
#1936
#2719
#3617

A reasonable approach to implement this is to use
https://github.com/rse/stmux
or
https://www.npmjs.com/package/@xterm/headless

Only the very last terminal in the chat history should be interactive.
Implementing this should also indirectly address various rendering issues where showing raw unfiltered terminal output in the console causes problems when terminal commands that use advanced terminal escape codes run.

[jacob314]

This has come up enough in other issues to justify a rating of P1.