SS-3 Fix PROXY TCP version validation

[vhafdal]

Summary

Fixes PROXY v1 TCP version validation in SmtpParser so TCP4 and TCP6 parsing only accepts the exact single-digit version token expected by each branch.

Root Cause

The TCP4/TCP6 parser checks used && when rejecting invalid tokens, which allowed invalid numeric versions such as TCP5 or multi-digit variants such as TCP46 to pass the version check and continue parsing.

Changes

• Require the TCP version token to be numeric, single-character, and equal to 4 for TCP4 parsing.
• Require the TCP version token to be numeric, single-character, and equal to 6 for TCP6 parsing.
• Add regression coverage for invalid PROXY TCP version inputs.

Validation

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj
• Upstream-based branch result: 120 passed, 1 skipped.

Jira

• SS-3

[vhafdal]

Superseded by consolidated PR #284.