Adding peptide after phosphate on the last position works wrong in sync mode on #6798

Steps to Reproduce

1. Go to Macro - Sequence mode (clean canvas)
2. Load from HELM: RNA1{R(A)P}|RNA2{P}|RNA3{R(U)P.R(U)}|PEPTIDE1{E}|PEPTIDE2{D.D.D.D}$RNA1,RNA3,2:pair-5:pair|RNA1,PEPTIDE1,3:R2-1:R1|PEPTIDE1,RNA2,1:R2-1:R1|PEPTIDE1,RNA3,1:pair-2:pair|PEPTIDE2,RNA1,4:R2-1:R1$$$V2.0
   
3. Turn it to edit mode (sync mode on) and add E peptide to the last position after phosphate (press E keyboard button)
   

Actual behavior
Standalone E peptide appears on the canvas

Expected behavior
No antisense E added

@ljubica-milovic says:

Per the original reqs the peptide should remain there (on the opposite side of the chain) and if the user connects it somehow to either sense (h-bond) or antisense (minus) it should not jump bellow after exiting the edit mode.
But later on we had a call and agreed not to add an antisense node if it is not connected to either sense or antisense from the beginning.

Versions

• Ketcher Version 3.2.0-rc.4 Build at 2025-03-25; 16:50:39
• Indigo Version 1.30.0-rc.4.0-g552c98211-wasm32-wasm-clang-19.0.0
• Chrome Version 134.0.6998.178 (Official Build) (64-bit)
• Win10

Issue found while testing: #6254