System doesn't create antisense monomers for unsplit nucleotide, just copy it as it is

[AlexeyGirin]

Steps to Reproduce

1. Go to Macro - Sequence mode (clean canvas)
2. Load from HELM: RNA1{[2-damdA].[5hMedC].[Super-G].[AmMC6T].[Super-T].[5Br-dU].[5NitInd].R(A)}$$$$V2.0
   
   
3. Select all and call context menu for the sequence and click Create RNA antisense strand option

Actual behavior
System doesn't create antisense monomers for unsplit nucleotide, just copy it as it is
Same problem with Create DNA antisense strand

Expected behavior
System creates antisense monomers for unsplit nucleotide

As per requirement:

1. When the user selects (flex and snake modes) any backbone chain (only R1-R2 connections) or any part of the backbone chain, right clicking anywhere on the selected monomers should give a drop-down menu with to option to "Create Antisense Strand", as long as one of the monomers in the backbone is a sugar connected to a "sense base" (R3-R1) (see table above).

and per requirement:

• An element can be an unsplit nucleotide.

Versions

• Ketcher Version 3.3.0-rc.1 Build at 2025-04-02; 09:36:56
• Indigo Version 1.31.0-rc.1.0-g859747ec7-wasm32-wasm-clang-19.0.0
• Chrome Version 135.0.7049.42 (Official Build) (64-bit)
• Win10

Issue found while testing: #6508