mail: stop PHPMailer waiting five minutes on a dead SMTP host

[t0ma5]

Summary

• PHPMailer's default Timeout is 300 seconds. A firewalled or mistyped SMTP host then holds a php-fpm worker for five minutes per osc_sendMail() call (and per a hung Mail Settings test).
• Cap connect and command waits at 15 seconds. Plugins can change it with the phpmailer_smtp_timeout filter; the value is clamped to 1–60.
• Restore the Mail Settings Send a test email button (the JS was still there; the markup was not) and abort that request after 25 seconds.

Test plan

• php tests/phpmailer-smtp-timeout.php (pins the 15s default, SMTP Timeout/Timelimit, and the 1–60 clamp)
• Admin → Settings → Mail server: Send a test email to a working SMTP host still delivers
• Point hostname at a black hole / closed port: the test fails in ~15–25s instead of hanging until the browser gives up

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