Preserve UTF-8 SMTP domains

[vhafdal]

Summary

Preserves UTF-8 domain names when SMTP command parsing materializes domain strings for HELO, EHLO, MAIL FROM, and RCPT TO.

Root cause

SmtpServer already tokenizes bytes >= 0x80 as text and advertises SMTPUTF8. The mailbox local-part path decoded with UTF-8, but the domain side still used the default ASCII helper. That meant internationalized
domains such as dæmi.is were not preserved after parsing.

Changes

• Decode HELO and EHLO domains with UTF-8.
• Decode mailbox domains with UTF-8 when creating parsed mailbox addresses.
• Add parser coverage for Icelandic examples:
  • EHLO dæmi.is
  • MAIL FROM:<þorsteinn@dæmi.is> SMTPUTF8
  • RCPT TO:<þorsteinn@dæmi.is>

Validation

• dotnet build src/SmtpServer/SmtpServer.csproj
• dotnet build src/SmtpServer.Tests/SmtpServer.Tests.csproj
• git diff --check

Full test execution was not run on my host because the tests target net8.0, while the installed runtimes are .NET 9 and .NET 10.

[vhafdal]

Superseded by consolidated PR #284.