SS-6 Wire NetworkBufferSize into connection pipes

[vhafdal]

Summary

• Wire NetworkBufferSize into the PipeReader created for each SMTP connection stream.
• Preserve the configured buffer size after STARTTLS swaps the underlying stream.
• Validate NetworkBufferSize values so non-positive sizes fail early.
• Add tests proving the configured size is observable at the pipe read layer.
• Document the fix in the changelog.

Verification

• DOTNET_ROLL_FORWARD=Major dotnet test src/SmtpServer.Tests/SmtpServer.Tests.csproj
  • Passed: 120, Skipped: 1

Notes

• The option is kept as public API and now matches its existing XML documentation: it controls the size of the buffer used when reading from the network stream.
• Existing warnings remain unrelated: MailKit NU1902, AuthCommand XML doc warning, and existing xUnit analyzer warnings.

[vhafdal]

Superseded by consolidated PR #284.