Hanging phosphate (or CHEM) should not be considered as antisense chain

[AlexeyGirin]

Steps to Reproduce

1. Go to Macro - Flex mode (clean canvas)
2. Load from HELM: RNA1{R(U)}|CHEM1{[4aPEGMal]}|RNA2{R(A)P.R(A)}|RNA3{P}$CHEM1,RNA1,1:pair-2:pair|CHEM1,RNA2,1:R2-1:R1|RNA2,RNA3,5:pair-1:pair$$$V2.0
   
3. Switch to Sequence mode

Actual behavior
System considers hanging phosphate (or CHEM) as antisense chain

Expected behavior
System considers hanging phosphate (or CHEM) as side chain

Case with CHEM: RNA1{R(U)}|CHEM1{[4aPEGMal]}|RNA2{R(A)P.R(A)}|CHEM2{[4aPEGMal]}$CHEM1,RNA1,1:pair-2:pair|CHEM1,RNA2,1:R2-1:R1|RNA2,CHEM2,5:pair-1:pair$$$V2.0

Versions

• Ketcher Version 3.1.0-rc.1 Build at 2025-02-04; 21:27:02
• Indigo Version 1.29.0-rc.1.0-g2e30df8f7-wasm32-wasm-clang-19.0.0
• Chrome Version 133.0.6943.54 (Official Build) (64-bit)
• Win10

Issue found while testing: #5942