urllib.request.HTTPDigestAuthHandler crashes on a malformed WWW-Authenticate digest challenge

Bug description:

urllib.request.AbstractDigestAuthHandler.retry_http_digest_auth parses a
server-controlled WWW-Authenticate: Digest ... challenge without guarding the
parse, so a malformed challenge raises a bare ValueError or IndexError out
of urlopen() instead of being declined:

import urllib.request
h = urllib.request.HTTPDigestAuthHandler(None)
req = urllib.request.Request("http://example.invalid/")
h.retry_http_digest_auth(req, "Digest realm")   # ValueError
h.retry_http_digest_auth(req, "Digest realm=")  # IndexError
h.retry_http_digest_auth(req, "Digest")         # ValueError

Digest realm / Digest realm= fail in parse_keqv_list; Digest fails in
auth.split(' ', 1). The handler should decline a challenge it cannot
parse (return None), as it already does for a 40x it cannot handle.

CPython versions tested on:

3.12, 3.13, 3.14, 3.15

Operating systems tested on:

macOS

Linked PRs

• gh-153793

Start at urllib.request.AbstractDigestAuthHandler.retry_http_digest_auth and reproduce the three malformed WWW-Authenticate challenges shown in the issue. Done means malformed Digest challenges return None instead of raising ValueError or IndexError; linked PR gh-153793 indicates work is already underway.