Networks and Sites: Add update_welcome_user_headers filter to wpmu_welcome_user_notification()

[irozum]

Adds a new update_welcome_user_headers filter to wpmu_welcome_user_notification(), letting developers customize the headers of the Multisite "account activation successful" welcome email — for example to send it as HTML (Content-Type: text/html) or override the From address — the same way update_welcome_user_email already lets them customize the message body. Previously the headers were built as a single hardcoded string with no filter hook, so a plugin could change the email content but not its headers. The header construction now builds a $message_headers array (still fully compatible with wp_mail(), which accepts either a string or an array of header lines) and runs it through the new filter before sending, mirroring the existing new_site_email array-filter pattern already used by wpmu_new_site_admin_notification() elsewhere in this file.

Trac ticket: https://core.trac.wordpress.org/ticket/49564

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementation, tests, and PR description. Reviewed by Igor Rozum.

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