MessageContent: replace overflow-hidden with overflow-x-clip to preserve sticky positioning

[hebo-agent]

Description

`MessageContent` currently uses `overflow-hidden` on its root div. Per CSS spec, `overflow: hidden` creates a scroll container, which breaks `position: sticky` for any descendants (e.g., code block action buttons rendered by streamdown or similar markdown renderers).

Proposal

Replace `overflow-hidden` with `overflow-x-clip` on the `MessageContent` root div.

Why `overflow-x-clip`?

• `overflow: clip` does not create a scroll container (unlike `overflow: hidden`), so `sticky` positioning continues to work for descendants
• It still clips wide content (long URLs, inline code) from overflowing the message bubble horizontally
• `overflow-x-clip` specifically targets the horizontal axis, leaving vertical overflow unaffected

Suggested change

```diff

• "... overflow-hidden text-sm",

• "... overflow-x-clip text-sm",
  ```

Context

We discovered this while debugging unclickable code block buttons in a chat UI. While the root cause turned out to be a separate issue with `sticky` + `pointer-events-none` in nested scroll containers, the `overflow-hidden` → `overflow-x-clip` change is independently valuable as a layout improvement that preserves sticky positioning for any future descendants that need it.

Browser support

`overflow: clip` is supported in all modern browsers (Chrome 90+, Firefox 81+, Safari 16+).