test-pull-request.yml: 'codegen' base-branch filter has no effect

[ptr727]

Summary

.github/workflows/test-pull-request.yml declares:

```yaml
on:
pull_request:
branches: [ main, develop, codegen ]
```

pull_request.branches filters on the base ref of the PR, not the head. Codegen PRs are opened with head codegen-main / codegen-develop targeting base main / develop, so the codegen entry only matches a PR whose target is the codegen branch — which the workflow doesn't open anywhere. The entry is inert in practice and slightly misleading.

Suggested fix

Drop the codegen entry:

```yaml
on:
pull_request:
branches: [ main, develop ]
```

Codegen PRs targeting main/develop are already covered by those two entries.

Context

Found while applying the template's pattern to ptr727/NxWitness — Copilot's review caught it after I had naïvely added codegen-main/codegen-develop to the list. Verified by reading the GitHub Actions docs on pull_request.branches and by checking that codegen PR base refs in the template are main/develop.

If you want the workflow to actually fire on pushes to the codegen branches (not PRs targeting them), that needs a push: trigger or a workflow on a different event — but I suspect that wasn't the intent.