[G26karthik]

Description

normalize_kraus documents three completely positive list forms that are accepted across the package: [K1, ..., Kr], [[K1], ..., [Kr]] and [[K1, ..., Kr]] (r > 2). The sys=2 fast path added to kraus_to_choi in #1643 does its own format check instead: anything that is not a flat list is read as [[A, B], ...] pairs. So [[K1], [K2], [K3]] raises IndexError, and [[K1, K2, K3]] is read as the single pair (K1, K2), which silently returns the Choi matrix of X -> K1 X K2^* and drops K3. The sys != 2 path goes through partial_channel, which already uses normalize_kraus, so only the default sys is affected.

Because is_completely_positive, choi_rank and other predicates call kraus_to_choi, they give wrong answers for these forms too. With K = [[[1, 0], [0, 0]], [[0, 1], [0, 0]], [[0, 0], [1, 0]] / 2] (flat list: CP, Choi rank 3):

master                                 this PR
[[K1], [K2], [K3]]
  kraus_to_choi  IndexError            equals flat-list result
  is_completely_positive  IndexError   True
  choi_rank      IndexError            3
[[K1, K2, K3]]
  kraus_to_choi  wrong matrix          equals flat-list result
  is_completely_positive  False        True
  choi_rank      1                     3

Changes

• Classify the input with normalize_kraus in the sys=2 fast path, as partial_channel, apply_channel and channel_dim already do. For the CP forms the left stack is reused as the right one, as before. Flat lists, stacked 3-D arrays and [[A, B], ...] pairs take the same path as before.
• Add a parametrized regression test for both wrapped forms, checked against the flat-list result. Both cases fail on master.

is_trace_preserving and dual_channel still do their own list dispatch, so I left them out of this PR to keep it to one concern (and away from the is_quantum_channel work in #1987). Happy to send that separately if you want it.

Checklist

• Use ruff for errors related to code style and formatting. (ruff check and ruff format --check clean)
• Verify all previous and newly added unit tests pass in pytest. (pytest toqito/channel_ops/tests/test_kraus_to_choi.py toqito/channel_ops/tests/test_partial_channel.py toqito/channel_props/tests/test_choi_rank.py toqito/channel_props/tests/test_is_completely_positive.py toqito/channels: 115 passed)
• Check the documentation build does not lead to any failures. (no docs changed, not run)

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✅ All modified and coverable lines are covered by tests.

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[G26karthik]

@vprusso CI is green on this one. It lets kraus_to_choi accept wrapped CP Kraus forms. Could you take a look when you have time?