System can't load HELM with inline SMILE with R group (can't save either)

**Steps to Reproduce**
1. Go to Macromolecules mode - Flex mode
2. Load from HELM: `PEPTIDE1{[dF].C.F.[dW].K.T.C.[*N[C@H](CO)[C@@H](C)O |$_R1;;;;;;;$|]}$PEPTIDE1,PEPTIDE1,2:R3-7:R3$$$`
(HELM taken from https://pubchem.ncbi.nlm.nih.gov/compound/Octreotide#section=Biologic-Description )

**Actual behavior**
System throws exception: `Convert error! Given string could not be loaded as (query or plain) molecule or reaction, see the error messages: 'molecule: allowRGroupOnRSite(): atom #0 is not an r-site'`

**Expected behavior**
Error message should be more clear indication absence of square brackets as a problem

**Environment details:**
- Ketcher Version 3.6.0-rc.1 Build at 2025-07-01; 19:04:05
- Indigo Version 1.34.0-rc.1.0-g7d0cdcdef-wasm32-wasm-clang-19.0.0
- Chrome Version 138.0.7204.97 (Official Build) (64-bit)
- Win10

Found while testing: https://github.com/epam/Indigo/issues/2958