fix/4159: Fix sync indexing all public meta when Post Search is disabled

[Sidsector9]

Description of the Change

• When the "Post Search" feature is disabled, $search->weighting is null because Search::setup() never runs. This caused filter_allowed_metas() to fall through to filter_allowed_metas_auto(), indexing all public meta.
• Fix by instantiating a fallback Weighting object when $search->weighting is unavailable, so get_meta_mode() still returns 'manual'.
• Apply the same fallback in filter_allowed_metas_manual() to prevent a fatal error when accessing $search_feature->weighting while Search is disabled.

Closes #4159

Changelog Entry

Fixed - sync indexing all public meta when Post Search is disabled

Credits

Props @burhandodhy @Sidsector9

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.