http.cookiejar.http2time raises ValueError instead of returning None for a strict-format fake month

Bug description

http.cookiejar.http2time() documents that it returns None for an
unrecognized date format, but its STRICT_DATE_RE fast path raises a raw
ValueError when the month field matches the pattern [JFMASOND][a-z][a-z]
yet names a month that does not exist:

>>> from http.cookiejar import http2time
>>> http2time('Wed, 09 Foo 1994 22:23:32 GMT')
Traceback (most recent call last):
  ...
ValueError: 'foo' is not in list
>>> http2time('Wed, 09 Feb 1994 22:23:32 GMT')   # a real month still works
760832612.0

The slower parser (_str2time) already handles the same input via
try/except and returns None; only the strict fast path leaks. It is
reachable from parse_ns_headers() through a cookie's expires= attribute,
e.g. Set-Cookie: x=y; expires=Wed, 09 Foo 1994 22:23:32 GMT.

gh-60385 previously established the "returns None, never raises" contract
for a different http2time code path; the later STRICT_DATE_RE fast path
reintroduced a leak on this separate path.

CPython versions tested on

3.13, 3.14, 3.15

Operating systems tested on

Linux, macOS

Linked PRs

• gh-153678

Python の http.cookiejar モジュールから始め、STRICT_DATE_RE の高速パスを通る http2time() を追跡し、無効な月に対する _str2time の処理と比較します。厳密形式の不正な月が例外を発生させずに None を返し、有効な月は引き続き正常に解析されれば、変更は完了です。Issue で説明されている parse_ns_headers() の cookie expiry パスを考慮してください。