Enforce soft_probe_prompt_cap in GCGCached probe

[precognitivem0nk]

Fixes #1562

GCGCached builds its full prompt list at class definition time (13 suffixes × 2 base prompts = 26 prompts) and never consults run.soft_probe_prompt_cap. This means setting a cap has no effect on how many prompts GCGCached issues.

This PR adds an __init__ method that calls self._prune_data(self.soft_probe_prompt_cap) after super().__init__(), following the same pattern used by DanInTheWild in dan.py. When the cap is set lower than the total prompt count, prompts are randomly sampled down to the cap.

Verified locally: with soft_probe_prompt_cap = 10, GCGCached now produces 10 prompts instead of 26. Added a test in tests/probes/test_probes_suffix.py to confirm the behavior.

[github-actions]

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[precognitivem0nk]

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