Convert containers assigned to BoxList items and slices

[Eric3-jp]

Replacing a BoxList element with a dictionary currently stores a plain dict, so attribute access stops working:

from box import BoxList

rows = BoxList([{"name": "before"}])
rows[0] = {"name": "after"}
rows[0].name  # AttributeError before this change; "after" afterward

Route ordinary item assignment, dotted list-index assignment, and slice assignment through the existing _convert() helper. This gives replacements the same recursive conversion and option propagation as append() and insert(). Slice assignments still accept iterators, and box_intact_types remains honored.

The regression tests cover positive/negative indices, dotted indices with default-list extension, dictionaries, nested lists, and both ordinary and extended slices. Compatibility tests cover intact types, invalid extended-slice lengths, and frozen lists. Includes the changelog and contributor updates requested by CONTRIBUTING.rst.

Validation on Ubuntu 24.04 (WSL), CPython 3.12.3:

• Before the implementation change: 10 failed, 3 passed in the new test selection.
• Full pure-Python suite: 172 passed.
• Cython 3.3.0 build via python setup.py build_ext --inplace: passed; full suite with the compiled box_list extension loaded: 172 passed.
• Black 24.10.0 with .black.toml: passed for both changed Python files.
• mypy 1.13.0: no issues in 8 source files.
• git diff --check: passed.

This change and its tests were prepared and validated with OpenAI Codex assistance.