test: add coverage for markdown chat message rendering

[afurm]

src/client/components/MarkdownMessage.tsx renders assistant output through react-markdown and remark-gfm, but it does not have focused component coverage yet.

Why this is a good starter task:

• The component is isolated and small.
• The expected behavior is visible in rendered DOM.
• It improves confidence in assistant/chat UI changes without touching provider logic.

Suggested scope:

• Add src/client/components/MarkdownMessage.test.tsx using the existing jsdom/Vitest component test style.
• Cover plain paragraphs, strong text, lists, fenced/inline code, links, and a GFM table or task list.
• Verify external links render with target="_blank" and rel="noreferrer".

Acceptance criteria:

• The new test file fails if the markdown component stops rendering common assistant output correctly.
• The test does not require network access or OpenAI credentials.
• npm test and npm run typecheck pass.