Change closed PR icon color from red to purple in Notifications view

[Copilot]

Closed (unmerged) PRs displayed with red checkmark icons in the Notifications view, causing confusion—users expected checkmarks to be green or at least not red. GitHub uses purple for closed PRs.

Changes

• Updated pullRequests.closed theme color from #cb2431 (red) to #8957e5 (purple) in both dark and light themes

This aligns with GitHub's UI convention and reuses the existing purple from pullRequests.merged, differentiated by icon shape (git-pull-request-closed vs git-pull-request).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Unclear what icons mean in Notifications view</issue_title> <issue_description>I'd expect anything with a checkmark to be green - I was confused why there are red checkmarks:

Clicking in, I realize this icon means a PR is closed, but a checkmark doesn't == closed issue to me.

I thought hovering over the icon might give me a tooltip on what the status means, but it only provides me the repo and issue number.

</issue_description>

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

@alexr00 It's the same icon that GitHub uses, but probably it should be purple.

</comment_new>

• Fixes microsoft/vscode-pull-request-github#7300

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