Fix copy/paste when Terminal Chat is open

[PankajBhojwani]

Summary of the Pull Request

TerminalPage was handling the copy/paste actions even when TerminalChat was open, preventing the command from reaching the palette. This commit checks the visibility of the chat pane before handling these actions.

Validation Steps Performed

Paste/copy are handled appropriately if the chat is open.

PR Checklist

• Closes Ctrl+V to paste in Terminal Chat goes to the active control instead #16589
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[carlos-zamora]

Curious, why do we have to check if Terminal Chat is open and not for the Command Palette? Can't we do the same thing the Command Palette did (however that was; maybe it was with the [preview]key-down/-up handlers?)