V2000 Mol file loader skips essential SGroup line in certain cases

[smz-bayer]

Summary
Reading a V2000 molfile that contains a line M SCN .... HT without the trailing space will skip the following molfile line if the M SCN line is not in an SGroup of type "SRU"

Steps to Reproduce

1. Use Indigo library with .NET wrapper in a .NET C# console application project.
2. Load attached molfile into string "mol":
   string mol = File.ReadAllText(@"C:\Temp\mul_with_scn_no_space_before_eol.txt");
3. Load that string into an Indigo molecule and write it back as another molfile string:
   using (var indigo = new com.epam.indigo.Indigo()) { string loadAndBack = indigo.loadMolecule(mol).molfile(); }

Actual behavior
The new molfile string now contains a molfile where an essential line is missing from the SGroup information:
That missing line is M SAL 1 3 2 4 5, and it was immediately following the line M SCN 1 1 HT

Expected behavior
The new molfile string should contain a complete molfile with correctly written SGroup information.
It might be slightly rearranged and have unneccessary information removed, but it must still contain the line M SAL 1 3 2 4 5

Environment details:

• Windows 11
• Indigo.NET nuget package 1.26.0 or 1.28.0-rc.2

Attachments
mul_with_scn_no_space_before_eol.txt

Additional context
According to V2000 MOL file spec, a line M SCN .... ends with a left justified 3-digit string.
However, all of the alternatives for that string only have 2 digits ("HH", "HT", or "EU").
Thus, the trailing space may be present or absent, still yielding a valid mol file.

The Indigo code has an extra check to deal with that, but the check only works if the M SCN ... line belongs to an SGroup of type "SRU".

In older molfiles you often find M SCN ... lines in other SGroup types, like "MUL". They may be removed without harm in such cases, but the rest of the SGroup information needs to stay consistent.