Add "Delete source branch" button to closed merge requests

What does this MR do and why?

This MR adds a "Delete Source Branch" button to the closed MR widget.

• Extracts the post-merge delete source branch logic into a reusable mixin sourceBranchRemovalMixin, and uses it in both MRWidgetMerged and MRWidgetClosed.
• Fixes a bug in the merged MR widget where the "Delete source branch" button was not removed after a successful deletion. The button now disappears immediately by setting mr.sourceBranchRemoved = true directly from the API response, rather than relying on MRWidgetUpdateRequested which triggered a GraphQL refetch that did not include sourceBranchExists.
• A new analytics event i_code_review_post_close_delete_branch is introduced to track usage separately from the post-merge equivalent.

References

• #1268
• #22134
• #225321

Screenshots or screen recordings

How to set up and validate locally

Use the following snippet to create 2 merge requests quickly:

project = Project.find_by_full_path('x')
user = project.first_owner
repo = project.repository
ts = Time.now.to_i

2.times do |i|
 branch_name = "test-delete-source-branch-#{ts}-#{i + 1}"

 repo.create_branch(branch_name, project.default_branch)

 repo.create_file(
 user,
 "test-#{ts}-#{i + 1}.txt",
 "test content #{i + 1}",
 message: "Add test file #{i + 1}",
 branch_name: branch_name,
 )

 mr = MergeRequests::CreateService.new(
 project: project,
 current_user: user,
 params: {
 title: "Test delete source branch #{i + 1}",
 source_branch: branch_name,
 target_branch: project.default_branch,
 force_remove_source_branch: '0',
 },
 ).execute

 puts "Created MR !#{mr.iid}: #{mr.title} (#{mr.source_branch})"
end

Test the change with a closed MR:

1. Close one of the merge requests
2. Wait a few moments for the MR widget to update - the "Delete Source Branch" button should appear.
3. Click the "Delete Source Branch" button - if successful the button will disappear, and you will see an XHR request to DELETE http://gdk.test:3000/<group>/<project>/-/branches/<branch> return a 200 / {"message":"Branch was deleted"}

To ensure that the change did not break existing functionality, test the change with a merged MR:

1. Merge one of the merge requests.
2. Wait a few moments for the MR widget to update - the "Delete Source Branch" button should appear.
3. Click the "Delete Source Branch" button - if successful the button will disappear, and you will see an XHR request to DELETE http://gdk.test:3000/<group>/<project>/-/branches/<branch> return a 200 / {"message":"Branch was deleted"}

MR acceptance checklist

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