🐛 Materialize iterable middleware arguments before inspecting them

[MiWeiss]

append_middleware/prepend_middleware are typed Iterable[Middleware], but the stack builders iterate the argument twice — once to compute middleware types for the duplicate warning, once to build the stack. A generator or other one-shot iterator is exhausted by the first pass, so the middleware is silently dropped.

import bibtexparser
from bibtexparser.middlewares import SeparateCoAuthors

bibtexparser.parse_string('@article{k, author={A and B}}', append_middleware=[SeparateCoAuthors()])
# ['A', 'B']  correct
bibtexparser.parse_string('@article{k, author={A and B}}', append_middleware=iter([SeparateCoAuthors()]))
# 'A and B'   wrong

Fix: materialize both arguments to lists at the top of _build_parse_stack and _build_unparse_stack, before anything inspects them. Public annotations stay Iterable; they are now actually honoured.

Tests: 9 regression tests over the public entry points (parse_string, parse_file, write_string, write_file) with generators. Suite: 2739 passed, 12 skipped (was 2730 + 12).

Fixes #531

🤖 Generated with Claude Code

[MiWeiss]

lgtm