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.

從聊天輸入框的圖片貼上處理開始，在剪貼簿中有圖片的情況下按住 Ctrl+V，以便在 Windows 上重現該問題。追蹤重複貼上事件的處理方式，然後驗證按住一次按鍵只會插入一張圖片，且正常的個別貼上仍然有效。

由索引模型根據 Issue 內容生成。