SS-2 Tighten SMTP parser syntax handling

[vhafdal]

Summary

Tightens SMTP parser handling for malformed known commands.

Root Cause

Some command parsers accepted a valid prefix and returned a command without verifying the command ended cleanly. MAIL FROM also treated malformed trailing ESMTP parameters as if no parameters were supplied. At the top-level TryMake path, syntax errors from recognized commands could be overwritten by later parser attempts and reported as an unrecognized command.

Changes

• Preserve syntax errors from recognized command parsers in top-level TryMake.
• Require HELO/EHLO parsed domains and address literals to be followed only by command end/whitespace.
• Reject malformed trailing MAIL FROM ESMTP parameters instead of silently using an empty parameter dictionary.
• Add regression tests for malformed HELO/EHLO/MAIL inputs and top-level syntax error reporting.

Validation

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj
• Result: 126 passed, 1 skipped.

Jira

• SS-2

[vhafdal]

Superseded by consolidated PR #284.