Fix extended slice assignment hanging on infinite iterators

[Copilot]

When assigning an arbitrary iterable to an extended slice (step != 1), use bounded iteration to collect at most slicelength + 1 items instead of eagerly consuming the entire iterable via PySequence_Fast. This prevents infinite iterators from hanging indefinitely.

Changes Made

• Objects/listobject.c: Added bounded iteration path for non-list/tuple iterables on extended slice assignment, with distinct error messages depending on the case:
  • Iterator yielded too many items: "attempt to assign iterable that yielded more items than extended slice of size N"
  • Iterator yielded too few items: "attempt to assign sequence of size N to extended slice of larger size"
  • List/tuple size mismatch (unchanged): "attempt to assign sequence of size X to extended slice of size Y"
• Lib/test/list_tests.py: Added tests for extended slice assignment with infinite generators, finite iterators (correct length, too few, too many), and error message validation
• NEWS entry: Added via blurb add for Attempting to assign an infinte generator to an extended slice hangs indefinitely instead of raising ValueError python/cpython#146268 attributed to Charles Machalow

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[csm10495]

@copilot see python#146272 (comment)

Fix that. The gist is that in this case the size is undetermined because it's an iterable it shouldn't say the size that we've gotten to this far but rather something along the lines of

ValueError: attempt to assign sequence of size 3 to extended slice of larger size.

Make minimal changes and update unit tests. Make sure at least one test ensures the correct exception wording for the situation.

[Copilot]

@copilot see python#146272 (comment)

Fix that. The gist is that in this case the size is undetermined because it's an iterable it shouldn't say the size ...

Updated the error messages in c4c841f. The bounded iteration path now uses distinct wording for each case:

• Too many items (including infinite): "attempt to assign iterable that yielded more items than extended slice of size %zd"
• Too few items: "attempt to assign sequence of size %zd to extended slice of larger size"
• List/tuple (exact sizes known): unchanged original message

Tests verify the correct exception wording for all three cases.

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