Use current document for mouse listeners

[anthonykim1]

For context and img description of the problem: microsoft/vscode#307196

• TL;DR: Selection does not work on CLIs such as Copilot CLI or Claude code on dedicated terminal window.
• Open via: workbench.action.terminal.newInNewWindow aka Terminal: New Terminal Window command in VS Code.

Changes in this PR:

• Attach transient protocol mouse listeners to the terminal element’s current ownerDocument, so mouse drag/release events work after the terminal moves into another window.
• Manage those transient listeners with MutableDisposables, so cleanup removes them from the exact document they were attached to without manually tracking the document.

After this PR: