[BUG] Expanded task header renders prompt as raw text (no markdown) and shows a second, inconsistently-styled scrollbar

[easonLiangWorldedtech]

Summary

Two related issues in the expanded task header (the collapsible "Task" panel at the top of an open task):

1. Prompt is not rendered as markdown. The collapsed title uses plain-text rendering, which is fine for a single-line summary, but the expanded view also used the same plain-text component, so prompts containing markdown syntax display it verbatim — e.g. bold markers, backtick code spans, lists and headings appear as literal characters while assistant messages in the same conversation render formatted markdown via MarkdownBlock.

2. Inconsistent / doubled scrollbars. The expanded prompt box (overflow-auto max-h-80) uses a default always-visible Chromium scrollbar, while the message list below (Virtuoso) uses the VS Code-style .scrollable hover-reveal scrollbar from index.css. For long prompts both bars are visible stacked in the same column with different styling.

Steps to Reproduce

1. Start a task whose prompt contains markdown (e.g. paste a markdown document).
2. Expand the task header (click the title / chevron).
3. Observe:
   • the prompt text shows raw markdown source instead of rendered formatting;
   • for long prompts, two scrollbars appear in the same column — one default-style (prompt box) and one VS Code-style (message list).

Expected Behavior

• The expanded task header renders the prompt through the same MarkdownBlock pipeline as chat messages.
• The prompt box uses the shared .scrollable class so its scrollbar matches the message list (transparent until hover/focus).

Suggested Fix

In webview-ui/src/components/chat/TaskHeader.tsx:

• Replace the plain-text Mention component in the expanded state with MarkdownBlock and drop the now-redundant whitespace-pre-wrap class.
• Add the .scrollable class to the prompt box so it inherits the VS Code scrollbar styling.

Additional Information

• Regression tests added in webview-ui/src/components/chat/tests/TaskHeader.spec.tsx (markdown rendering + scrollable class).
• Verified: vitest 23/23 passing, ESLint clean, tsc -b passes.