fix(frontend/copilot): constrain markdown heading sizes in user chat messages (#12463)

### Before

<img width="600" height="489" alt="Screenshot 2026-03-18 at 19 24 41"
src="https://github.com/user-attachments/assets/bb8dc0fa-04cd-4f32-8125-2d7930b4acde"
/>

Formatted headings in user messages would look massive

### After

<img width="600" height="549" alt="Screenshot 2026-03-18 at 19 24 33"
src="https://github.com/user-attachments/assets/51230232-c914-42dd-821f-3b067b80bab4"
/>

Markdown headings (`# H1` through `###### H6`) and setext-style headings
(`====`) in user chat messages rendered at their full HTML heading size,
which looked disproportionately large in the chat bubble context.

### Changes 🏗️

- Added Tailwind CSS overrides on the user message `MessageContent`
wrapper to cap all heading elements (h1-h6) at `text-lg font-semibold`
- Only affects user messages in copilot chat (via `group-[.is-user]`
selector); assistant messages are unchanged

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [ ] Send a user message containing `# Heading 1` through `######
Heading 6` and verify they all render at constrained size
- [ ] Send a message with `====` separator pattern and verify it doesn't
render as a mega H1
  - [ ] Verify assistant messages with headings still render normally

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -207,6 +207,7 @@ export function ChatMessagesContainer({
                 className={
                   "text-[1rem] leading-relaxed " +
                   "group-[.is-user]:rounded-xl group-[.is-user]:bg-purple-100 group-[.is-user]:px-3 group-[.is-user]:py-2.5 group-[.is-user]:text-slate-900 group-[.is-user]:[border-bottom-right-radius:0] " +
+                  "group-[.is-user]:[&_h1]:text-lg group-[.is-user]:[&_h1]:font-semibold group-[.is-user]:[&_h2]:text-lg group-[.is-user]:[&_h2]:font-semibold group-[.is-user]:[&_h3]:text-lg group-[.is-user]:[&_h3]:font-semibold group-[.is-user]:[&_h4]:text-lg group-[.is-user]:[&_h4]:font-semibold group-[.is-user]:[&_h5]:text-lg group-[.is-user]:[&_h5]:font-semibold group-[.is-user]:[&_h6]:text-lg group-[.is-user]:[&_h6]:font-semibold " +
                   "group-[.is-assistant]:bg-transparent group-[.is-assistant]:text-slate-900"
                 }
               >