Networks and Sites: Add admin email to Sites list table

[Sukhendu2002]

Adds an Admin Email column to Network Admin → Sites using each site's admin_email option.

The column is initially hidden through Screen Options, including for users with existing column preferences. A cold benchmark measured 21 additional queries for 20 visible rows, so the lookup only runs after explicit opt-in.

The internal site_admin_email key avoids intercepting plugin-defined admin_email columns handled by manage_sites_custom_column.

Trac ticket: https://core.trac.wordpress.org/ticket/33832

Tests cover:

• Column registration and output.
• No option lookup while hidden.
• Existing Screen Options preferences.
• Persistence after enabling the column.
• Compatibility with custom admin_email columns.

AI assistance: Yes
Tool(s): Pi coding agent
Model(s): GPT-5.6
Used for: Ticket validation, initial test skeleton, and code review.Final implementation and tests were reviewed and edited by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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