String with invalid syntax can be imported

Found while testing #3541

**Steps to Reproduce:**

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

**Actual Result:**
A monomer with the name `###` is added to the canvas

**Expected Result:**
Only valid BILN strings (with correct codes and syntax) should be imported. Invalid strings should not be imported, and an error message should be displayed:
"The string cannot be interpreted as a valid BILN string."

As per Req:

As per BA:

**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 149.0.7827.156 (Official Build) (64-bit)
- Win11

Reproduce the issue by pasting `###` into Macromolecules - Flex mode, then using the `Open as New` and `Add to Canvas` entry points. Trace how the BILN string is handled and verify that invalid syntax is rejected with the specified error while valid BILN strings remain importable.

Written by the indexing model from the issue text.