Improve SMTP protocol support, streaming paths, benchmarks, and conformance

[vhafdal]

Summary

This draft PR aggregates the SMTP Server Jira work completed on nexer-prod from the fork. It is intended as a single review path for the protocol, streaming, benchmark, parser, limit, and EHLO conformance improvements instead of opening more dependent PRs.

Included Work

• Tighten SMTP parser syntax handling for malformed known commands.
• Fix PROXY TCP4/TCP6 version validation.
• Add streaming DATA ingestion through IStreamingMessageStore while preserving existing IMessageStore behavior.
• Add verb-based parser dispatch and parser benchmarks.
• Wire NetworkBufferSize into connection pipe setup.
• Reduce AUTH/EHLO allocations and add focused benchmarks.
• Add DSN envelope parameter parsing and parameter-aware recipient/filter surfaces.
• Add HELP, VRFY, EXPN command handling and policy extension points.
• Add CHUNKING/BDAT support, including multi-chunk transactions and streaming store support.
• Add enhanced SMTP status codes for common replies.
• Add benchmark coverage for parser, AUTH/EHLO, DATA, BDAT, and throughput paths.
• Reduce buffered BDAT fallback copying by avoiding a final whole-message ToArray() where possible.
• Harden parser support for multi-segment command buffers.
• Add MaxCommandLineLength separate from MaxMessageSize.
• Document and test EHLO extension advertisement behavior.

Notes For Reviewers

• The existing IMessageStore.SaveAsync(... ReadOnlySequence<byte> ...) contract is preserved.
• Streaming storage is opt-in via IStreamingMessageStore.
• MaxMessageSize remains focused on DATA/BDAT message content; command and AUTH continuation lines now use MaxCommandLineLength.
• EHLO extension documentation now states behavior and limitations for PIPELINING, 8BITMIME, SMTPUTF8, SIZE, DSN, CHUNKING, STARTTLS, and AUTH.
• This branch currently includes work that also exists in earlier smaller PRs from the fork; this consolidated PR can replace those if preferred.

Validation

Most recent verification on the final nexer-prod merge state:

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj
  • Passed: 196
  • Skipped: 1
• Benchmark slices were run for the benchmark/BDAT allocation work, including before/after BDAT allocation checks.

Existing warnings observed during local validation:

• NU1900 package vulnerability lookup warnings when package sources were unreachable from the environment.
• Existing package readme warning.
• Existing xUnit analyzer warnings in pre-existing tests.

Jira

Covers SS-2 through SS-16, including the final consolidated work for SS-12, SS-13, SS-14, SS-15, and SS-16.

[vhafdal]

So I would not have to wait a long time for PR approval I decided to just do one clean PR that includes all the changes.
Not sure how long you take in approving, but I will just create a custom nuget package and keep going from there and try to track what we can add later on.