[LDAP] Send welcome mail for new users from LDAP/AD backend

[!TIP]

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Is your feature request related to a problem? Please describe.
Currently, when new users are synchronized or provisioned via the LDAP/AD backend (user_ldap), Nextcloud does not trigger welcome emails. While local users created via the UI or Provisioning API can automatically receive a welcome email based on the core app configuration newUser.sendEmail, LDAP users are completely excluded from this onboarding flow.

As an administrator, there is no out-of-the-box way to inform newly synced LDAP users that their account is ready to use, forcing admins to rely on custom scripts or external mailings.

Describe the solution you'd like
Support the existing newUser.sendEmail for newly mapped LDAP users.

1. When a new user is created from user_ldap, check if newUser.sendEmail is set to yes.
2. Trigger a welcome email that dynamically adapts to read-only backends. If a user belongs to a backend that does not support password changes, the welcome email should omit the "Set your password" link/token and instead inform the user that they can log in using their standard LDAP/AD credentials.

Describe alternatives you've considered

• Keep as-is, users are not directly informed.
• Manually click "re-send welcome mail"

Additional context

• none

Start by tracing new-user creation in the user_ldap backend and the existing core newUser.sendEmail flow used by local users and the Provisioning API. Check how the re-send welcome mail handles password-setting links and read-only backends. Done means newly mapped LDAP users receive the configured welcome mail, with password instructions matching their backend capabilities.