docs: document channel input errors

[happykawayigt]

Description

Documents explicit input-validation exceptions in two channel APIs as part of #1888.

Changes

• Document the invalid sys and empty Kraus-list cases in kraus_to_choi.
• Document rectangular-space, dimension-mismatch, inconsistent Kraus-size, and invalid dim cases in channel_dim.

Checklist

• Use ruff for errors related to code style and formatting.
• Verify all previous and newly added unit tests pass in pytest.
• Check the documentation build does not lead to any failures.

Validation

• Targeted tests: 28 passed
• ruff check
• ruff format --check
• git diff --check

A full local MkDocs build was not run because the documentation dependency group is not installed in this worktree.

AI assistance

Implemented and reviewed with OpenAI Codex; all listed validation commands were run locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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