`N-methylation` option is missing even if monomer in selection is eligible for it

[AlexeyGirin]

Confirmed by @ljubica-milovic

Steps to Reproduce

1. Go to Macromolecules mode - Snake mode (empty canvas!) <-- IMPORTANT!
2. Load from HELM: PEPTIDE1{P.P}$$$$V2.0
   
3. Select all monomers on the canvas and open context menu and hover mouse over Modify amino acids option

Actual behavior
N-methylation option is absent

Expected behavior
N-methylation option is present in the second level context menu because one of monomers is eligible for N-methylation

As per requirement:

2.2. A modification should appear if at least one amino-acid has that modification available.

Environment details:

• Ketcher Version 3.5.0-rc.3 Build at 2025-06-13; 18:57:13
• Indigo Version 1.33.0-rc.3.0-gded053799-wasm32-wasm-clang-19.0.0
• Chrome Version 137.0.7151.104 (Official Build) (64-bit)
• Win10

Found while testing: #5208

[AlexeyGirin]

[SefaBilgii]

This issue is reproducible

Ketcher Version: 3.6.0-rc.1
OS: MacOs Sequoia Version 15.5
Browser : Chrome Version 138.0.7204.169