• Closes #xxxx
• This PR follows our AI policy

Assisted-by: Codex:gpt-5.6-Sol

Summary

Match CPython's error reporting when dict() or dict.update() receives an invalid sequence element.

• Add a contextual exception note when an element cannot be converted to a sequence, enabling test_update_type_error.
• Report the zero-based element index and actual sequence length when an element does not contain exactly two items.

Implementation details

Previously, merge_from_seq2 extracted the first two items and only checked whether a third item existed. As a result, it could only raise the generic error:

Iterator must have exactly two elements

CPython converts each dictionary update element to a sequence before validating its length. This allows it to report both the index of the failing element and its actual length:

dictionary update sequence element #0 has length 3; 2 is required

This change follows the same approach:

• Exact PyList and PyTuple instances use their underlying slices directly, avoiding an unnecessary allocation.
• Other iterable objects are collected into a Vec so that their actual length can be determined.
• Non-iterable elements raise TypeError: object is not iterable.
• TypeErrors raised while converting or iterating an element are preserved and receive the contextual note:

Cannot convert dictionary update sequence element #0 to a sequence

PyList is imported to support the exact-list fast path, corresponding to CPython's PySequence_Fast handling of exact lists and tuples.

Summary by CodeRabbit

• Bug Fixes
  • Improved dictionary updates from sequences by consistently validating that each item contains exactly two elements.
  • Improved error messages for invalid or non-iterable update items, including contextual details identifying the problematic item.
  • Added optimized handling for list- and tuple-based update entries while preserving support for other iterables.