SS-5 Add verb-based SMTP parser dispatch

[vhafdal]

Summary

• Dispatch SmtpParser.TryMake by the first command verb instead of trying every command parser in sequence.
• Preserve existing command-specific parser methods and command factory creation behavior.
• Add top-level dispatch tests across supported verbs and unknown command behavior.
• Add SmtpParserBenchmarks and switch the benchmark runner to BenchmarkSwitcher for targeted runs.

Verification

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj
  • Passed: 128, 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 *SmtpParserBenchmarks*

Benchmark Notes

ShortRun parser benchmark on .NET 9.0.17:

• Unknown command: 1,013.37 ns -> 37.90 ns, allocation 1856 B -> 64 B
• AUTH: 1,959.90 ns -> 1,093.24 ns, allocation 4368 B -> 2960 B
• MAIL: 1,325.70 ns -> 1,099.03 ns, allocation 2528 B -> 2336 B
• RCPT: 922.43 ns -> 627.18 ns, allocation 1560 B -> 1240 B
• PROXY: 2,658.60 ns -> 1,662.89 ns, allocation 4984 B -> 3384 B
• EHLO remains effectively neutral: 287.07 ns -> 285.70 ns

[vhafdal]

Superseded by consolidated PR #284.