fix(network): do not split single-valued and http-date headers on comma

[Mohan Ram (mohanram-dev)]

Description

Fixes #42687.

On Firefox and WebKit, response headers are provided joined or parsed through internal comma splitting (ffNetworkManager.ts and headersObjectToArray).

Per RFC 9110 §5.3, field lines containing an HTTP-date (Date, Expires, Last-Modified, If-Modified-Since, If-Unmodified-Since, Retry-After) and standard single-valued headers (Location, Content-Type, Content-Disposition, etc.) are not comma-separated lists. Splitting on , corrupted every date header (e.g., splitting "Wed, 21 Oct 2026 07:28:00 GMT" into "Wed" and "21 Oct 2026 07:28:00 GMT"). Additionally, on macOS WebKit where wkSetCookieSeparator = ',', cookies with Expires attributes containing commas were being split within the date string.

This PR:

1. Defines singleValuedHeaders Set in packages/isomorphic/headers.ts containing standard single-valued and HTTP-date header names.
2. Updates headersObjectToArray to preserve single-valued headers intact without splitting on ,.
3. Adds splitSetCookieString to safely handle comma-separated Set-Cookie headers while preserving dates inside Expires=... attributes.
4. Updates ffNetworkManager.ts (parseMultivalueHeaders) to avoid splitting single-valued and date headers on commas.
5. Adds comprehensive unit tests (tests/library/unit/headers.spec.ts) and end-to-end response header tests (tests/page/page-network-response.spec.ts).