Allow scrolling through the current conversation history

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

● Describe the feature or problem you'd like to solve ┃ In Copilot CLI, using the mouse wheel or PageUp/PageDown navigates through prompt history instead of scrolling the ┃ current conversation. ┃ ┃ When the conversation becomes long, I cannot scroll up to review earlier messages at the top of the same ┃ conversation. This makes it difficult to refer back to previous requirements, responses, and context.

### Proposed solution

Provide a way to scroll through the current conversation independently from prompt-history navigation.
Mouse-wheel scrolling and PageUp/PageDown should scroll the conversation view, while prompt-history navigation
should use separate configurable keybindings.

### Example prompts or workflows

_No response_

### Additional context

1. Have a long conversation with multiple prompts and responses.
2. Scroll upward to review messages from earlier in the same conversation.
3. Use PageUp/PageDown to move through the conversation history.
4. Use separate keys when navigating previously entered prompts.

Start in the Copilot CLI input and conversation-view handling for mouse-wheel and PageUp/PageDown events. Trace how those keys currently navigate prompt history, then identify the existing keybinding configuration; done means scrolling reviews earlier messages in the current conversation while prompt-history navigation uses separate configurable keys.

Written by the indexing model from the issue text.