fix(core): skip malformed cookie fragments in mergeCookies

[anxkhn]

mergeCookies() splits each source cookie string on ; and forced the result
of Cookie.parse() with a non-null assertion:

const cookie = Cookie.parse(cookieString)!;

tough-cookie's Cookie.parse returns undefined (it does not throw) for a
fragment it cannot parse, for example a bare name with no = (sessionid) or
one that starts with =. The ! suppressed that undefined, so the very next
line dereferenced it and threw:

TypeError: Cannot read properties of undefined (reading 'key')

This runs on the navigation hot path: http-based crawlers call mergeCookies
for every request that carries a cookie header, and that header can be an
arbitrary string a user set on the Request. A single malformed fragment turns
into a confusing crash that repeats on every retry until the request is marked
failed, instead of the fragment simply being ignored.

The fix guards the parsed cookie and skips fragments that cannot be parsed,
matching the two existing defensive continues already in the same loop. Valid
cookies are unaffected.

const cookie = Cookie.parse(cookieString);
// ignore fragments that tough-cookie cannot parse (e.g. a bare name with no value)
if (!cookie) continue;

Testing

Added a regression test next to the existing mergeCookies() test in
test/core/crawlers/cheerio_crawler.test.ts. It fails on master with the
TypeError above and passes with the fix:

test('mergeCookies() skips malformed cookie fragments instead of throwing', () => {
    expect(mergeCookies('https://example.com', ['valid=1; brokenfragment'])).toBe('valid=1');
    expect(mergeCookies('https://example.com', ['a=b', 'c'])).toBe('a=b');
    expect(mergeCookies('https://example.com', ['sessionid'])).toBe('');
});

yarn vitest run test/core/crawlers/cheerio_crawler.test.ts -> 48 passed.
Lint (eslint), typecheck (tsc --noEmit -p packages/core/tsconfig.json), and
biome format are all clean.

[barjin]

Thank you for your interest in this project @anxkhn !

The change you're describing is breaking - Crawlee would now silently swallow errors about invalid cookies. I believe we want to keep the throwing behaviour.

If you think the error message is confusing or doesn't correctly describe the source of the error, I'd prefer to update the error handling (e.g., share more information about the error in the message). What do you think?