fix: case-insensitive ConfigBox key lookup for __getitem__, __contains__, __delitem__, __setitem__

[gaoflow]

ConfigBox advertises "loosey goosey" case-insensitive key access in its docstring,
but:

• __getitem__ (dict-style cb['host']) had zero case-insensitive support — only exact match
• __contains__ ('host' in cb) was not overridden — False for differently-cased keys
• __delitem__ and __setitem__ likewise had no case-insensitive support
• __getattr__ only retried with .lower(), missing MixedCase→lowercase resolution

Fix: Added _case_insensitive_key() static helper that scans actual keys
case-insensitively, then overrode the five dunder methods to use it. Each
method tries exact match first (zero overhead for exact-case), then falls back
to case-insensitive scan.

Reproduction:

from box import ConfigBox
cb = ConfigBox({'HOST': 'localhost', 'ServerPort': 8080})
cb['host']          # BoxKeyError — should return 'localhost'
'serverport' in cb  # False — should be True
del cb['HOST']      # BoxKeyError — should succeed

154/154 tests pass (5 toon-related failures pre-existing, missing toon_format).

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