Add Open on GitHub action to PR editors

[Copilot]

PR editors can remain stuck loading, leaving no way to open the pull request externally.

Editor action

• Adds the globe-based Open Pull Request on GitHub action to PR editor toolbars.

{
  "command": "pr.openPullRequestOnGitHub",
  "when": "activeWebviewPanelId == 'PullRequestOverview'"
}

Loading-safe URL resolution

• Targets the active PR editor.
• Uses the loaded PR URL when available.
• While loading, reconstructs the URL from the retained PR identity and matching GitHub host, including GitHub Enterprise.

• Fixes #332009

[azure-pipelines]

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[Copilot]

Pull request overview

This PR adds an “Open Pull Request on GitHub” action to the PR overview editor toolbar and implements loading-safe URL resolution so users can open the PR externally even when the webview is stuck in a loading state.

Changes:

• Added a new editor/title toolbar action (pr.openPullRequestOnGitHub) scoped to the PR overview webview.
• Added PullRequestOverviewPanel.getCurrentPullRequestUrl() to return the loaded PR URL when available, or reconstruct it from retained PR identity + matching repository remote while loading.
• Refactored the command handler so it can open the active PR editor’s URL (or fall back to choosing from active PRs).

Show a summary per file

File	Description
src/test/github/pullRequestOverview.test.ts	Adds coverage for resolving an active PR URL before the PR model finishes loading.
src/github/pullRequestOverview.ts	Implements loading-safe URL resolution via getCurrentPullRequestUrl().
src/commands.ts	Refactors the command to support editor-toolbar invocation and active-panel URL fallback.
package.json	Contributes the PR editor toolbar action gated by activeWebviewPanelId == 'PullRequestOverview'.

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• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite