fix: export emails to CSV is not working

[sapayth]

Summary

Fixes the "Save Emails to CSV File" tool, which was producing an empty file (only the header row, no subscriber data). The export now correctly writes every selected subscriber, and the downloaded file is no longer corrupted by unrelated PHP notices or HTML-encoded characters.

Security Considerations

• Email addresses submitted for export are now individually validated and sanitized before being processed, in addition to the existing capability and nonce checks.
• Output is isolated so the CSV file contains only subscriber data, preventing incidental content from other plugins or themes from leaking into the download.