Add "Request review from Copilot" button to Reviewers section

[Copilot]

GitHub's web interface now provides a shortcut to request reviews from Copilot. This PR adds the same capability to the VS Code extension.

Changes

Backend (src/github/pullRequestOverview.ts):

• Added addReviewerCopilot handler that calls requestReview() with Copilot user
• Cached assignable users in _assignableUsers to avoid redundant API calls
• Added canRequestCopilotReview property computed from cached users and existing reviewers

Frontend (webviews/components/sidebar.tsx):

• Added Copilot icon button in Reviewers section (mirrors Assignees pattern)
• Button conditionally renders when canRequestCopilotReview && !isCopilotAlreadyReviewer
• Handles ITeam vs IAccount type checking with isITeam() guard

Types (src/github/views.ts):

• Extended PullRequest interface with canRequestCopilotReview: boolean

Button appears only when user has write permissions, Copilot is available, and not already a reviewer. Loading state prevents duplicate requests.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Request review from Copilot</issue_title> <issue_description>In the Github Pull request web interface, there is now a shortcut button to request a review from the Copilot bot.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes microsoft/vscode-pull-request-github#6830

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.