fix: preserve existing keys on dotted list-element writes

[gaoflow]

Summary

With default_box=True and box_dots=True, writing a second dotted key into an already-existing list element silently discards the keys already written there:

from box import Box
b = Box(default_box=True, box_dots=True)
b["a[0].x"] = 1     # {'a': [{'x': 1}]}
b["a[0].y"] = 2
b.to_dict()         # {'a': [{'y': 2}]}   <-- 'x' is gone, expected {'a': [{'x': 1, 'y': 2}]}

The write succeeds but drops previously-stored data — a silent correctness/data-loss bug. (Different indices like a[0].x then a[1].y are fine; the bug is a second write to the same index.)

Cause

BoxList.__setitem__ (box/box_list.py), in the default_box branch, unconditionally replaces the element at the target index with a fresh empty Box/BoxList before recursing into it — even when a populated container already lives there. So the second write to a[0] throws away the Box({'x': 1}) already stored and recurses into a brand-new empty one.

The sibling Box.__setitem__ already guards this with an "is it already the right container?" check; the BoxList path was missing it.

Fix

Read the current element first and only replace it when it isn't already the right container type. A scalar (or None placeholder from list extension) is still correctly overwritten; an existing Box/BoxList is preserved and recursed into.

Added regression coverage to test_box_list_default_dots (two keys into one element, nested list-in-list, and the scalar-overwrite case). Full suite: 154 passed; the 5 test_toon_* failures are pre-existing (a NotImplementedError/BoxError from the third-party toon_format package) and reproduce identically on a clean master. black --config=.black.toml --check clean.

This pull request was prepared with the assistance of AI, under my direction and review.