Validate and normalize partition_all batch sizes

[muneikh]

partition_all(0, rows) and negative sizes silently produce an empty iterator even with nonempty input. A valid integer-index object is accepted when the iterator group is built, but can raise TypeError later when a short final batch requires arithmetic on the original object.

Normalize the batch size once using operator.index and require it to be positive before touching the source. Document the requirement and test empty/nonempty inputs, unchanged source consumption on invalid size, and an indexable size with a short final batch.

Validation: three regressions fail against the original code; the library and doctest suite passes with the fix (271 passed, 2 skipped). Configured pycodestyle and git diff --check pass. Nonpositive sizes now raise ValueError when the generator is advanced; non-integral sizes remain TypeError.