chore: Delete copilot/* branches in E2E test workflow (#19)

- [x] Update "Clean up issues and pull requests" step in main branch's
test.yml to delete PR branches
- [x] Test bash logic for correctness and edge cases
- [x] Code review completed and addressed
- [x] Address feedback:
- Move branch extraction after `gh pr close` (closer to where it's used)
  - Move `|| true` inside `$()` for proper error handling
  - Add `TESTING_REPOSITORY` environment variable to avoid hardcoding
  - Use environment variable in both the scan step and branch deletion
- Use `gh api` instead of `git push` for consistent authentication
context

The implementation:
- Extracts branch name from each PR using `gh pr view "$URL" --json
headRefName -q .headRefName || true`
- Deletes the branch after closing the PR using `gh api -X DELETE
"repos/${{ env.TESTING_REPOSITORY }}/git/refs/heads/$branch"`
- Handles failures gracefully with `|| true` inside command substitution
- Uses centralized `TESTING_REPOSITORY` environment variable for
consistency
- Uses `gh api` for branch deletion to maintain consistent
authentication context with other `gh` CLI commands

Note: Similar changes will need to be applied to v1 and v2 branches
separately after this PR is merged.

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[Task] Delete `copilot/*` branches in E2E test
workflow</issue_title>
> <issue_description>### Problem
> 
> Before transferring the action to
[github/accessibility-scanner](https://github.com/github/accessibility-scanner),
I deleted thousands of `copilot/*` branches by running `git branch -r |
grep -Eo 'copilot/.*' | xargs -I {} git push origin :{}`. This took more
than 30 minutes to complete.
> 
> Because of
github-community-projects/continuous-ai-for-accessibility-scanner#2202
[^1], the new `github/accessibility-scanner` repo will never have
thousands of defunct `copilot/*` branches…but
`github/accessibility-scanner-testing` will. It’s [already
starting](https://github.com/github/accessibility-scanner-testing/branches/all?query=copilot%2F).
> 
> ### Solution
> 
> - In [`main`’s
test.yml](https://github.com/github/accessibility-scanner/blob/main/.github/workflows/test.yml#L95-L121),
[`v2`’s
test.yml](https://github.com/github/accessibility-scanner/blob/v2/.github/workflows/test.yml#L95-L121),
and [`v1`’s
test.yml](https://github.com/github/accessibility-scanner/blob/v1/.github/workflows/test.yml#L137-L163)
update the “Clean up issues and pull requests” step to also delete the
branch for each PR it deletes.
> - Do not use `git branch -r | grep -Eo 'copilot/.*' | xargs -I {} git
push origin :{}` to delete all Copilot branches—that is too broad, and
it could delete branches unrelated to E2E testing (i.e. branches we want
to keep).
> - The `gh` CLI doesn’t have a command or flag for deleting branches
alongside PRs, but something like this should work: `branch="$(gh pr
view <PR-number> --json headRefName -q .headRefName)" && [ -n "$branch"
] && git push origin --delete "$branch" || true`.
> 
> [^1]: And
https://github.com/github/accessibility-scanner/pull/7/files#diff-faff1af3d8ff408964a57b2e475f69a6b7c7b71c9978cccc8f471798caac2c88
and
https://github.com/github/accessibility-scanner/pull/8/files#diff-faff1af3d8ff408964a57b2e475f69a6b7c7b71c9978cccc8f471798caac2c88</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

Fixes github/continuous-ai-for-accessibility#76

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for
you](https://github.com/github/accessibility-scanner/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
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Author: smockle

.github/workflows/test.yml

@@ -17,6 +17,9 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: false # Allow previous workflows’ clean-up steps to complete
 
+env:
+  TESTING_REPOSITORY: github/accessibility-scanner-testing
+
 jobs:
   test:
     name: Test
@@ -74,7 +77,7 @@ jobs:
           login_url: http://127.0.0.1:4000/
           username: ${{ secrets.TEST_USERNAME }}
           password: ${{ secrets.TEST_PASSWORD }}
-          repository: github/accessibility-scanner-testing
+          repository: ${{ env.TESTING_REPOSITORY }}
           token: ${{ secrets.GH_TOKEN }}
           cache_key: ${{ steps.cache_key.outputs.cache_key }}
 
@@ -110,6 +113,11 @@ jobs:
                 if [[ "$URL" == *"/pull/"* ]]; then
                   echo "Closing pull request: $URL"
                   gh pr close "$URL" || echo "Failed to close pull request: $URL"
+                  branch="$(gh pr view "$URL" --json headRefName -q .headRefName || true)"
+                  if [[ -n "$branch" ]]; then
+                    echo "Deleting branch: $branch"
+                    gh api -X DELETE "repos/${{ env.TESTING_REPOSITORY }}/git/refs/heads/$branch" || echo "Failed to delete branch: $branch"
+                  fi
                 elif [[ "$URL" == *"/issues/"* ]]; then
                   echo "Closing issue: $URL"
                   gh issue close "$URL" || echo "Failed to close issue: $URL"