Iterator passed as append_middleware/prepend_middleware is silently dropped

[MiWeiss]

The parameters are typed Iterable[Middleware], but _build_parse_stack consumes the iterable when computing append_stack_types before calling list(append_middleware) (bibtexparser/entrypoint.py:36-45; same pattern in _build_unparse_stack, lines 66-75). A generator/iterator therefore results in an empty middleware list — silently.

lib = bibtexparser.parse_string("@article{x, month = {3}}",
    append_middleware=iter([MonthIntMiddleware()]))
lib.entries[0]["month"]  # '3' -- middleware was never applied

Fix: materialize to a list at the top of the builders (or type the params as List).

[claell]

I opened draft PR #569 with the focused fix and public parse/write iterator regressions. The branch is based directly on current main and its full test suite passes.