fix: isolate few-shot sampler RNG and pool from shared and global state (#1307, #1309)

[Abelo9996]

What

Two fixes in FewShotSampler (src/lighteval/tasks/prompt_manager.py) that leak state and break variance-seed reproducibility.

Sequential (#1307)

_init_fewshot_sampling_sequential rotated the list returned by task.fewshot_docs() in place. That list is the task's memoized _fewshot_docs, handed back by reference, so the rotation mutated shared state and each cached seed aliased the same over-rotated list. With few_shot_iterations > 1, offsets accumulate and every seed after the first selects the wrong examples. Fix: copy the pool with list(...) before rotating, the same way _init_fewshot_sampling_random already does.

Balanced (#1309)

_init_fewshot_sampling_balanced had two issues:

1. It seeded and used the global random module, so selection depended on and perturbed global RNG state. Fixed with a local random.Random(variance_seed).
2. The label loop guarded with if not next_label, which also fires on a present but falsy label (int 0, empty string), cutting the balanced selection short. Fixed to stop only on next_label is None, which is the real "cycle exhausted" signal.

Behaviour

random.Random(seed) produces the same sequence as random.seed(seed) on clean state, so selections are unchanged in the common case. The change fixes the cases where shared or global state leaked between seeds, and where a falsy label truncated the balanced sample.

Tests

• New tests/unit/prompt/test_fewshot_sampler.py: sequential does not mutate the shared pool, sequential seeds are independent, a falsy label does not truncate the balanced sample, and balanced leaves global RNG state untouched.
• Updated test_fewshot_sampler[sequential] in tests/unit/prompt/test_prompt_manager.py: its previous assertion passed only because the in-place rotation mutated the shared pool the assertion also read from; it now checks the intended rotation ([20:40]).
• tests/unit/prompt/ is green; ruff check and ruff format --check pass on the changed files.

Closes #1307
Closes #1309