Fix sequential few-shot sampling mutating the shared few-shot pool

[ErenAta16]

Fixes #1307.

FewShotSampler._init_fewshot_sampling_sequential rotated the list returned by task.fewshot_docs() in place to offset the selection per seed. That list is the task's memoized _fewshot_docs, returned by reference, so the rotation mutated shared state. When an evaluation runs several few-shot seeds against the same task (variance estimation), each seed rotated the already-rotated pool, so the offsets accumulated and every seed after the first selected the wrong examples. The per-seed cache stores a reference rather than a copy, so the cached selections all aliased one over-rotated list, and the pool itself was left rotated after the run.

The fix copies the pool before rotating, matching what _init_fewshot_sampling_random already does. Only the sequential method is affected: random copies the pool, and balanced builds its own per-label lists and never mutates it.

Repro on main before the fix:

seed 0: picked=['0', '1']  expected=['0', '1']
seed 1: picked=['2', '3']  expected=['2', '3']
seed 2: picked=['6', '7']  expected=['4', '5']
pool after sampling: ['6', '7', '8', '9', '0', '1', '2', '3', '4', '5']
pool unchanged: False

Tests

The existing test_fewshot_sampler[sequential] compared the result against fewshot_docs() after sampling, so it read back the mutated pool and passed despite the bug. I updated it to check the rotation against the original order, and added two regression tests:

• test_sequential_fewshot_does_not_mutate_shared_pool: the memoized pool is unchanged after sampling.
• test_sequential_fewshot_seeds_are_independent: each seed selects the correct rotation from the original order.

Both new tests fail on main and pass with the fix.

pytest tests/unit/prompt/test_prompt_manager.py