`ask_user` tool should allow Ctrl-G to support long freeform answers

### Describe the feature or problem you'd like to solve

To support multi-paragraph long freeform answers, allow the user to compose a response to the `ask_user` tool in their $EDITOR

### Proposed solution

When describing situations to Copilot CLI, it is often helpful to provide additional context and information.

Currently, if an `ask_user` is going in the wrong direction, the user can either type in the freeform answer spot, which works poorly if more than few lines, for example, #3722, or the user can Ctrl-C to kill the `ask_user` tool, then hit Ctrl-C a second time to stop Copilot from continuing, and then finally type Ctrl-G to enter into $EDITOR to compose the desired response to the `ask_user`.

Additionally, since the user had to leave the `ask_user` tool with Ctrl-C, if the user later decides they want to respond to the `ask_user` tool with a non-freeform answer, the option is gone since the tool is no longer running.

The user experience would be improved if Ctrl-G were allowed _during_ an `ask_user` tool call.

### Example prompts or workflows

Here's a screenshot of an example of the issue:

### Additional context

_No response_

Start at the ask_user tool's input handling and inspect the existing Ctrl-G and $EDITOR flow in Copilot CLI. Confirm that Ctrl-G opens the editor during an active ask_user call, while non-freeform response options remain available afterward; add or run the relevant interaction tests if present.

Written by the indexing model from the issue text.