Add "Convert to draft" action in PR reviewers section

[Copilot]

Users can mark a draft PR as ready for review but cannot revert it back to draft. This adds symmetry to the workflow.

Changes

GraphQL layer

• Added ConvertToDraft mutation in queriesShared.gql
• Added ConvertPullRequestToDraftResponse interface

Model

• Implemented convertToDraft() method in PullRequestModel with telemetry

UI

• Added "Convert to draft" button in reviewers section (sidebar)
• Button shown when PR is not draft AND (user is author OR has write permission)
• State management handles busy/disabled states during conversion

Message routing

• Added pr.convertToDraft command handler in pullRequestOverview.ts
• Added convertToDraft() context method in webview layer

Implementation

Mirrors the existing "Ready for review" pattern:

// Backend
async convertToDraft(): Promise<ConvertToDraft> {
  const { data } = await mutate<ConvertPullRequestToDraftResponse>({
    mutation: schema.ConvertToDraft,
    variables: { input: { pullRequestId: this.graphNodeId } }
  });
  this.item.isDraft = data.convertPullRequestToDraft.pullRequest.isDraft;
  return result;
}

// Frontend component
{!pr.isDraft && (hasWritePermission || pr.isAuthor) && (
  <ConvertToDraft />
)}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support "Convert to draft" in PR view</issue_title>
<issue_description>I created a PR and then realized it wasn't ready. It would be great if I could convert the PR to draft from vscode.

Especially since we can mark a PR ready for review:

(not a very important FR, of course)</issue_description>

<agent_instructions>Add a "convert to draft" in the "reviewers" section of the PR webview.</agent_instructions>

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

• Fixes Support "Convert to draft" in PR view #6028

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