Samples only set once

Branch: develop/1.1

https://github.com/Azure/counterfit/blob/ff4f3c7c593856830e5a9b07bd1627f71fbcfc61/counterfit/core/attacks.py#L51

Getting samples happens in `cfattack.__init__`. It should happen in `cfattack.prepare_attack` which is called before the attack is run in `core.run_attack`.

```python
# Get the samples.
self.samples = target.get_samples(
self.options.cf_options["sample_index"]["current"]
)
```

Start in counterfit/core/attacks.py at cfattack.__init__ and inspect prepare_attack, then trace the call from core.run_attack. Confirm samples are obtained during attack preparation rather than only during initialization, and verify the attack flow uses the current sample index.

Written by the indexing model from the issue text.