Support RCPT ESMTP parameters

[vhafdal]

Summary

Adds parser and command support for optional ESMTP parameters on RCPT TO commands.

Changes

• Parse trailing ESMTP parameters after the RCPT path.
• Expose parsed recipient parameters on RcptCommand.Parameters.
• Add a compatibility overload to ISmtpCommandFactory.CreateRcpt so existing implementations that only override CreateRcpt(IMailbox) continue to work.
• Update SmtpCommandFactory and command tracing to include RCPT parameters.
• Add parser tests for valid DSN-style parameters and invalid parameter syntax.

Validation

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

dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj --filter "FullyQualifiedName~SmtpParserTests" --no-build could not execute on this host because the test project targets net8.0 and this machine only has .NET 9/10 runtimes installed.

Existing warnings remain: XML docs in AuthCommand, the package missing-readme warning, MailKit vulnerability warning, and existing xUnit analyzer warnings.

[vhafdal]

Superseded by consolidated PR #284.