Bootstrap/Load: Send HTTP headers after querying posts in WP::main().
By running WP::send_headers() after posts have been queried, we can ensure that conditional tags like is_front_page(), is_home(), etc. work as expected.
This provides better context and more flexibility when adjusting HTTP headers via the wp_headers filter or send_headers action.
Previously, the earliest action where conditional tags worked correctly was wp.
Includes moving the X-Pingback header, previously sent in WP::handle_404() after posts have been queried, to a more appropriate place in WP::send_headers().
Follow-up to [2627], [34442].
Props jonoaldersonwp, joostdevalk, peterwilsoncc, adamsilverstein, SergeyBiryukov.
Fixes #56068.