Monomers without hyphens in square brackets in BILN string are loaded incorrectly

Found while testing #3541

Note: after fixing this issue, requirement 6 of Import section with the provided examples in https://github.com/epam/Indigo/issues/3541 should be re-tested.

**Steps to Reproduce:**

1. Open Macromolecules - Flex mode (clean canvas)
2. Paste the following BILN string from the clipboard: `[D]-[meF]-[G]`
3. Click `Open as New` or `Add to Canvas` button

**Actual Result:**
Monomers are loaded incorrectly with square brackets in their names

**Expected Result:**
Monomers are loaded correctly without square brackets in their names

**Found in Environment:**
- Ketcher Version 3.17.0-rc.1 Build at 2026-06-02; 21:27:10
- Indigo Version 1.45.0-rc.2.0-g168fb85f2-wasm32-wasm-clang-23.0.0
- Chrome Version 149.0.7827.103 (Official Build) (64-bit)
- Win11

Start by reproducing the BILN import in Macromolecules - Flex mode with `[D]-[meF]-[G]`, then trace the import handling that produces the monomer names. Verify that the loaded names omit square brackets, and re-test requirement 6 from issue #3541 with its provided examples.

Written by the indexing model from the issue text.