Multisite: Pass network object instead of ID to `pre_get_main_site_id`.

In the original function introduced in [41380], and subsequently [41861], only the network ID was passed to the new `pre_get_main_site_id` filter. In order to make give quick access to other network properties, it was decided to pass the whole object instead. The changeset includes an additional test.

Fixes #29684.

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git-svn-id: https://core.svn.wordpress.org/trunk@41877 1a063a9b-81f0-0310-95a4-ce76da25c4cd