http.cookies.BaseCookie.load() applies partial state before raising CookieError

Bug description

BaseCookie.load() parses the cookie string in two phases: parse then apply. The comment says it rejects the string if syntactically invalid. However, _is_legal_key() validation runs during the apply phase (inside Morsel.set()), not the parse phase. Cookies before an illegal key are applied to the jar before CookieError is raised.

from http.cookies import SimpleCookie, CookieError

C = SimpleCookie()
try:
    C.load("a=1; b,c=2; d=3")
except CookieError:
    pass
print(len(C))  # 1, expected 0

Related to gh-84183 but distinct: that issue is about raise-vs-silent for different invalid inputs; this is about partial state when CookieError IS raised.

CPython versions tested on

3.13, 3.14, main

Operating systems tested on

macOS

Linked PRs

• gh-154674

Start at http.cookies.BaseCookie.load() and reproduce the issue with the cookie string shown in the report. Verify that a CookieError leaves the cookie jar empty; the issue body links PR gh-154674, so check that work before proceeding.