Add encoding validation in parse_file() and fix write_file parameter bug

- Validate the encoding parameter before attempting to open the file.
  This provides a clearer error message when an invalid encoding is
  specified, rather than letting the error propagate from the open() call.
- Fix bug from PR #489 where write_file() referenced undefined variables
  (parse_stack/append_middleware) instead of the renamed parameters
  (unparse_stack/prepend_middleware).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MiWeiss

bibtexparser/entrypoint.py

@@ -1,3 +1,4 @@
+import codecs
 import warnings
 from typing import Iterable
 from typing import List
@@ -125,7 +126,14 @@ def parse_file(
 
     :param encoding: Encoding of the .bib file. Default encoding is ``"UTF-8"``.
     :return: Library: Parsed BibTeX library
+    :raises LookupError: If the specified encoding is not recognized.
     """
+    # Validate encoding before opening file to provide clearer error message
+    try:
+        codecs.lookup(encoding)
+    except LookupError:
+        raise LookupError(f"Unknown encoding: {encoding!r}")
+
     with open(path, encoding=encoding) as f:
         bibtex_str = f.read()
         return parse_string(