System should be able to load unknown monomer on any position #3265

Steps to Reproduce

1. Open Macromolecules - Flex mode (clean canvas)
2. Load from AxoLabs: 5'-(Unknown1)(Unknown2)(Unknown3)-3'

Actual behavior
System throws an error: Convert error! Given string could not be loaded as (query or plain) molecule or reaction, see the error messages: 'molecule auto loader: SMILES loader: invalid character within atom description: '''', 'scanner: BufferScanner::read() error', 'Ket Document: Template with id '' already added', 'molecule auto loader: SMILES loader: invalid character within atom description: '''', 'molecule auto loader: SMILES loader: invalid character within atom description: '''', 'scanner: BufferScanner::read() error'

Expected behavior
System loads sequence of three unknown monomers

As per requirements:

4. In case of import, where a symbol with brackets that is not defined in the Ketcher library exist, the monomer should be loaded as an unresolved AxoLab monomer (see ticket)

Environment details:

• Ketcher Version 3.9.0-rc.1 Build at 2025-10-16; 19:08:29
• Indigo Version 1.37.0-rc.2.0-gef86e7136-wasm32-wasm-clang-19.0.0
• Chrome Version 141.0.7390.55 (Official Build) (64-bit)
• Win10

Related issue: #3152