SS-7 Reduce AUTH and EHLO allocations

[vhafdal]

Summary

• Stream EHLO response lines without materializing an intermediate array.
• Replace AUTH PLAIN regex parsing with explicit NUL-delimited parsing over decoded bytes.
• Use non-throwing base64 decode paths for AUTH PLAIN and LOGIN invalid inputs.
• Emit a single authentication failure response for invalid AUTH PLAIN input.
• Add raw SMTP tests for invalid AUTH base64 and stable EHLO output.
• Add Auth/EHLO benchmarks and enable filtered benchmark runs.

Verification

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj --no-restore
  • Passed: 121, Skipped: 1
• DOTNET_ROLL_FORWARD=Major dotnet build src/SmtpServer.Benchmarks/SmtpServer.Benchmarks.csproj
  • Passed with existing MailKit NU1902 advisory warning
• DOTNET_ROLL_FORWARD=Major dotnet run -c Release --project src/SmtpServer.Benchmarks/SmtpServer.Benchmarks.csproj -- --filter *AuthEhloBenchmarks*

Benchmark Notes

ShortRun on .NET 9.0.17:

• EHLO: 311.9 ns, 528 B allocated
• AUTH PLAIN: 327.8 ns, 248 B allocated
• Invalid AUTH PLAIN: 239.0 ns, 136 B allocated

[vhafdal]

Superseded by consolidated PR #284.