Accept array-style keys in from_query_string

[gaoflow]

Problem

from_query_string rejects valid query strings that use array-style keys, which urllib.parse.parse_qs parses without complaint:

from benedict import benedict
benedict.from_query_string("a[]=1&a[]=2")        # ValueError: Invalid query string
benedict.from_query_string("user[name]=joe")     # ValueError: Invalid query string
benedict.from_query_string("items[0]=x&items[1]=y")  # ValueError: Invalid query string

The decode gate in QueryStringSerializer used a hardcoded character whitelist ([\w\-\%\+\.\|]) for keys and values, which omits [ and ] (and other valid characters), so any bracketed/array key fails the gate before parse_qs is ever called. Array-style keys are common (PHP and HTML-form syntax).

Fix

Replace the opaque whitelist with a structural check: split on & and require every segment to be key=value where the key is non-empty and neither side contains raw whitespace, = (key) or &. This accepts bracketed/array keys while still rejecting the other formats benedict's autodetect must distinguish (verified against the repo's own fixtures: TOML, YAML, JSON, XML, plain text and URLs are still rejected).

Tests

Added cases for array-style (a[]=1&a[]=2) and bracketed (user[name]=joe&user[age]=42) keys via both IODict.from_query_string and IODict(s, format="query_string"). Without the fix they raise ValueError; with it the query-string tests pass and the broader IO/serializer suite is green (279 passed locally). ruff check/format clean.

Disclosure: I used AI assistance (Claude) to help locate and draft this fix, under my direction and review.