fix(emails): Add more email logging

[vbudhram]

Because

• Email sending failures with "501 Invalid RCPT TO address provided" errors don't include which recipient addresses are being rejected
• Debugging SMTP errors requires more context about rejected vs accepted recipients
• CC recipients aren't logged in error cases, making it harder to identify problematic addresses

This pull request

• Adds SMTP response code and message to error logs in EmailSender and auth-server mailer
• Logs rejected and accepted recipient addresses when email sending fails
• Includes CC recipients in error log details
• Adds test coverage for SMTP error logging with recipient details
• Enhances retry attempt logs with SMTP response information

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12710

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information

[nshirley]

I'm curious if there's any reason to not just include this with a null coalescence?

        this.log.warn('mailer.send.retry', {
          err: err.message,
          to: email.to,
          template: email.template,
          retryAttempt: attempt,
          nextAttempt: attempt + 1,
          backoffMs: Math.round(backoffDelay),
          // additional, optional values
          smtpResponse: err.response ?? 'not set',
          smtpResponseCode: err.responseCode ?? 'not set',
        });

[vbudhram]

No big reason, I do kind of prefer the latter since it doesnt include in the log 😅

[nshirley]

Looks good! Just one minor, non-blocking question