fix(tests): stabilize flaky Hub LFS integration test
## Problem

`test_push_dataset_dict_to_hub_overwrite_files` intermittently fails with:
```
BadRequestError: LFS pointer pointed to a file that does not exist
```

This has been causing the `deps-latest` integration tests to fail on main (visible in recent CI runs). I ran into this while working on the BIDS loader PR and dug into the root cause.

## Root Cause

Two race conditions in the test:

1. **LFS propagation timing** - Rapid successive `push_to_hub` calls don't wait for Hub to fully propagate LFS objects between pushes
2. **Repo name reuse** - The second test scenario reused the same repo name from scenario 1, creating a race between deletion and recreation

## Solution

- Add `_wait_for_repo_ready()` helper that polls `list_repo_files` to ensure the repo is consistent before subsequent operations
- Use a unique repo name (`ds_name_2`) for the second scenario, eliminating the delete/create race entirely

## Testing

All 4 integration test variants now pass:
- ✅ `ubuntu-latest, deps-latest` (was failing)
- ✅ `ubuntu-latest, deps-minimum`
- ✅ `windows-latest, deps-latest` (was failing)
- ✅ `windows-latest, deps-minimum`

Validated on fork: https://github.com/The-Obstacle-Is-The-Way/datasets/pull/4

## Related

- #7600 (push_to_hub concurrency)
- #6392 (push_to_hub connection robustness)

cc @lhoestq - small fix but should help CI reliability