Holding Ctrl+V pastes the same image repeatedly (no debounce/repeat guard)

### Description
When pasting an image into the chat input, holding down `Ctrl+V` fires the paste event continuously with no debounce or key-repeat guard. Holding it for ~10 seconds inserts ~100 copies of the same image into the conversation.

### Version
GitHub Copilot CLI 1.0.69-0 (desktop)

### OS
Windows

### Steps to reproduce
1. Copy an image to the clipboard.
2. Focus the chat input.
3. Press and hold `Ctrl+V` for several seconds.

### Expected behavior
A held `Ctrl+V` should paste the image once (or be debounced / ignore key auto-repeat), like a normal single paste.

### Actual behavior
The image is pasted repeatedly (one insert per key-repeat event), flooding the chat with dozens of duplicate images.

Start at the chat input's image-paste handling and reproduce the issue on Windows by holding Ctrl+V with an image on the clipboard. Trace how repeated paste events are processed, then verify that one held keypress inserts only one image and that normal separate pastes still work.

由索引模型根据 Issue 内容生成。