feat(copilot): friendlier 'Response stopped' banner (#13114)

## Why

The previous cancellation message ('You manually stopped this chat' with
an exclamation icon) felt like an error state rather than a neutral user
action. The mockup calls for a softer, more polished treatment.

## What

- Replaced `ExclamationMarkIcon` with `StopIcon` (filled, neutral-400)
from `@phosphor-icons/react`
- Updated copy from 'You manually stopped this chat' → **'Response
stopped'**
- Adjusted banner styling to match design system: `bg-neutral-50`,
`border-neutral-200`, `px-4 py-3`, `gap-2`

## Where


`autogpt_platform/frontend/src/app/(platform)/copilot/components/ChatMessagesContainer/components/MessagePartRenderer.tsx`

<img width="853" height="335" alt="Screenshot 2026-05-15 at 12 40 44 PM"
src="https://github.com/user-attachments/assets/c2792523-b177-4c26-bc74-af95030c211b"
/>

## Checklist
- [x] Uses Phosphor Icons only (`StopIcon` from `@phosphor-icons/react`)
- [x] No `dark:` Tailwind classes
- [x] No new dependencies
- [x] Matches mockup design

---
*Addresses feedback: use the attached mockup to make this error message
more user-friendly when someone stops a message.*

---------

Co-authored-by: abhi1992002 <abhimanyu1992002@gmail.com>
Co-authored-by: Ubbe <hi@ubbe.dev>