fix: Don't set defaults when parsing cookies from headers

[elliott-with-the-longest-name-on-github]

Closes #9901

Bypasses setting Kit defaults on cookies parsed from fetch response headers. To illustrate what the problem was:

• Set a cookie in +server.ts: cookies.set('foo', 'bar', { httpOnly: false })
• fetch this endpoint from +page.server.ts
• As part of fetch, parse the cookies from the headers
• httpOnly isn't set (because false === unset)
• Adding this cookie to the cookies causes the default httpOnly: true to be added

The better solution here is not to apply defaults to cookies we parse from fetch response headers. If these cookies were set through cookies.set in a +server.ts endpoint, we've already set the defaults, and if they're not, we shouldn't touch them anyway.

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[Rich-Harris]

I think this is the right general approach, yeah (though I don't think we need with_defaults).

It does pain me somewhat to have an unexpected property on the cookies object. It may not be in the type definition but it's still visible to users, and people will abuse implementation details if we expose them. Can we instead return set_internal as a standalone function that is passed to create_fetch (like get_cookie_header)?