Configuring SMTP server for no-authentication access

[Copilot]

Pull request created by AI Agent

[Copilot]

Pull request overview

This PR updates the SMTP mail-sending logic to support SMTP servers that allow sending without SMTP AUTH, by only attempting authentication when both username and password are configured.

Changes:

• Introduced a _useSmtpAuthentication flag derived from configured username/password.
• Made both the background sender worker and the SMTP connectivity test skip Authenticate* calls when auth is not configured.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.35%. Comparing base (7041e21) to head (8c72312).

Files with missing lines	Patch %	Lines
src/GZCTF/Services/Mail/MailSender.cs	33.33%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             work     #586      +/-   ##
==========================================
- Coverage   59.36%   59.35%   -0.02%     
==========================================
  Files         190      190              
  Lines        9804     9811       +7     
  Branches     1202     1204       +2     
==========================================
+ Hits         5820     5823       +3     
- Misses       3488     3491       +3     
- Partials      496      497       +1     

Files with missing lines	Coverage Δ
src/GZCTF/Services/Mail/MailSender.cs	11.51% <33.33%> (+1.38%)	⬆️

... and 2 files with indirect coverage changes

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