gh-154546: Make http.cookies.BaseCookie.load() atomic on error

[gianghungtien]

BaseCookie.load() is documented as parse-then-apply — the code even comments that it "first parse[s] the whole cookie string and reject[s] it if it's syntactically invalid". However, the key and attribute legality checks actually run during the apply phase (inside Morsel.set() and Morsel.__setitem__()), not the parse phase. So when an illegal key or an unknown $-prefixed attribute appears part-way through the string, CookieError is raised only after the earlier cookies have already been written into the jar, leaving it in a partially-updated state.

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 on main, expected 0

This builds the morsels in a temporary mapping and only commits them to the jar once the whole string has validated, so load() is now all-or-nothing. Existing morsels are copied before being updated, so a failing load() no longer mutates cookies that were already present in the jar. This also covers the sibling case of an unknown $-prefixed attribute (e.g. "a=1; $foo=2"), which had the same partial-application problem.

Regression tests and a Misc/NEWS.d entry are included.

Fixes #154546

• Issue: http.cookies.BaseCookie.load() applies partial state before raising CookieError #154546