ci: Use date-based branch names for toxgen PRs (#5704)

Reusing the `toxgen/update` branch can get annoying, especially since it
needs manual babysitting (empty commit to trigger CI). Let's use
distinct names instead.

## Summary
- Instead of always reusing the same `toxgen/update` branch, the "Update
test matrix" workflow now creates a date-based branch (e.g.
`toxgen/update-03-19`).
- Same-day reruns force-push to the same branch, so they overwrite
rather than fail.
- The PR-closing logic now finds and closes all open PRs from any
`toxgen/` branch, not just the current one.

## Test plan
- [ ] Trigger the "Update test matrix" workflow manually and verify it
creates a branch like `toxgen/update-MM-DD`
- [ ] Trigger it again on the same day and verify it overwrites the
branch and closes the previous PR
- [ ] Verify old toxgen PRs from different branches get closed

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sentrivana

.github/workflows/update-tox.yml

@@ -42,7 +42,7 @@ jobs:
         run: |
           COMMIT_TITLE="ci: 🤖 Update test matrix with new releases"
           DATE=`date +%m/%d`
-          BRANCH_NAME="toxgen/update"
+          BRANCH_NAME="toxgen/update-`date +%m-%d`"
 
           git checkout -B "$BRANCH_NAME"
           git add --all
@@ -78,20 +78,21 @@ jobs:
 
             // Close existing toxgen PRs as they're now obsolete
 
-            const { data: existingPRs } = await github.rest.pulls.list({
+            const existingPRs = await github.paginate(github.rest.pulls.list, {
                 owner: context.repo.owner,
                 repo: context.repo.repo,
-                head: `${context.repo.owner}:${branchName}`,
                 state: 'open'
             });
 
             for (const pr of existingPRs) {
-                await github.rest.pulls.update({
-                    owner: context.repo.owner,
-                    repo: context.repo.repo,
-                    pull_number: pr.number,
-                    state: 'closed'
-                })
+                if (pr.head.ref.startsWith('toxgen/')) {
+                    await github.rest.pulls.update({
+                        owner: context.repo.owner,
+                        repo: context.repo.repo,
+                        pull_number: pr.number,
+                        state: 'closed'
+                    })
+                }
             };
 
             const { data: pr } = await github.rest.pulls.create({