Show the rendered description for REST-listed PRs in the overview - #9152

A PR opened from the "All Open" category showed its description as an empty card. That category is often the only place grouped Dependabot PRs appear. Since #8919, the overview renders the model it is given without fetching it again. Models listed through REST pulls.list have bodyHTML undefined, and CommentBody rendered bodyHTML ?? '' as HTML.

Changes

• Extension host: load the rendered description after the page is shown (pullRequestOverview.ts)

  • After pr.initialize, if the model has a body but no bodyHTML, the new loadDescription loads the PR through GraphQL (getPullRequest(..., 'overview')).
  • It posts pr.update with { body, bodyHTML }, processed by processLinksInBodyHtml like the initial render.
  • The result is dropped if the panel was disposed or another update started (_updateSequence), the same check stack loading uses.
  • getPullRequest updates the cached model shared with the tree, so other views get the rendered description too.

• Webview: show text instead of an empty body (comment.tsx)

  • bodyHTML === undefined (not loaded): render body as plain React text in a pre-wrap div, so it is not interpreted as HTML.
  • bodyHTML === '' (GitHub rendered nothing, e.g. a body that is only an HTML comment template): render empty, as before.
  • CommentPreview had its own copy of this fallback; it now calls CommentBody, so the preview and the full view render the same markup.

  const renderedBody = bodyHTML === undefined
  	? <div style={{ whiteSpace: 'pre-wrap' }}>{body}</div>
  	: <div dangerouslySetInnerHTML={{ __html: bodyHTML }} />;

Notes for review

• Opening a REST-listed PR now costs one extra GraphQL request after the page is shown. Opening still isn't blocked.
• The GraphQL update also fills in mergeable, which REST leaves unknown, so the panel reloads the stack and branch status once more. No refresh loop: bodyHTML is defined afterwards.
• stubPanelServices in pullRequestOverview.test.ts now stubs repo.getPullRequest, keeping its rule that repo.query is never called. Two existing tests that stubbed getPullRequest themselves now override the shared stub.
• New overview host tests have not been run locally (the VS Code test download is blocked in the sandbox), so they rely on CI.

• Fixes Dependabot PR description is blank in VS Code but renders correctly on GitHub #9063