gh-130110: allow hyphens in RFC 2231 continuation parameter names

decode_params() matches RFC 2231 continuation parameter names with the
regex r'^(?P<name>\w+)\*...'. That rejects names containing a hyphen
(e.g. `file-name*0*`), leaving such continuations undecoded.

Widen the name class to [\w-] so hyphenated names are recognized, while
still restricting to RFC 2045 `token` characters: under re.ASCII, \w is
[A-Za-z0-9_], all of which are valid token chars, so no invalid
character can match.

Co-Authored-By: Hermes Agent <noreply@nousresearch.com>

Author: Phantom-d-e-v

Lib/email/utils.py

@@ -401,6 +401,12 @@ def encode_rfc2231(s, charset=None, language=None):
 
 rfc2231_continuation = re.compile(r'^(?P<name>[\w-]+)\*((?P<num>[0-9]+)\*?)?$',
     re.ASCII)
+# `\w` is ASCII-only here (re.ASCII), so it matches exactly [A-Za-z0-9_],
+# all of which are valid RFC 2045 `token` characters. Adding `-` lets the
+# regex recognize hyphenated continuation parameter names (e.g. `file-name*0*`).
+# This is intentionally stricter than the full RFC 2045 `token` set (which
+# also permits e.g. `.`, `!`, `#`); none of those appear in practice for
+# RFC 2231 parameter names, and broadening the class is out of scope here.
 
 def decode_params(params):
     """Decode parameters list according to RFC 2231.