ci: avoid branch clash in regenerate-models workflow (#812)

## Summary

Fixes the branch-clash bug in
`.github/workflows/manual_regenerate_models.yaml` that was flagged as a
follow-up in #809. The recent manual run ([failing
CI](https://github.com/apify/apify-client-python/actions/runs/26239209577/job/77220728000))
failed at the `Commit model changes` step with:

```
fatal: refusing to fetch into branch 'refs/heads/update-models-manual' checked out at '...'
```

### Cause

The local `git switch -c "$BRANCH"` step pre-created the branch
unconditionally. `apify/actions/signed-commit@v1.0.0` with
`create-branch: 'true'` then ran `git fetch --depth=1 origin
"$BRANCH:refs/heads/$BRANCH"`, which refuses to fetch into a checked-out
branch.

### Fix

- When the remote branch already exists: fetch + check it out locally,
and tell signed-commit `create-branch: false`.
- When it doesn't exist: stay on the default branch and let
signed-commit create the remote branch with `create-branch: true`.

The `create_branch` decision is exported from the renamed `Set up
branch` step and consumed by the signed-commit step.