Make InvalidNameError copyable and picklable

[eeshsaxena]

InvalidNameError can't be copied or pickled, which crashes normal parsing:

import copy
from bibtexparser.middlewares.names import InvalidNameError
copy.deepcopy(InvalidNameError(name="Doe, John", reason="Too many commas"))
# TypeError: InvalidNameError.__init__() missing 1 required positional argument

Its __init__ takes name and reason but the base ValueError only keeps the formatted message in args, so deepcopy/pickle rebuild it with a single argument. This surfaces through the public API: a failed name is stored in a MiddlewareErrorBlock, and a later middleware like SortBlocksByTypeAndKeyMiddleware deepcopies the whole library, so any bad author name plus that middleware raises TypeError out of parse_string. I store name/reason and add __reduce__ so it round-trips. Added a test.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (Make InvalidNameError copyable and picklable), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining python-bibtexparser!