It is possible to export amino acid without BILN alias to BILN format

Found while testing #3541

**Pre-condition:**
Amino acid without BILN alias is present in the Monomer Library (e.g., created by a user)

**Steps to Reproduce:**

1. Open Macromolecules - Flex mode (clean canvas)
2. Drag the amino acid monomer from the precondition on the canvas
3. Click `Save as` button
4. Select BILN as the file format
5. Click `Save `button

**Actual Result:**
It is possible to export amino acid without BILN alias to BILN format, no error appears

https://github.com/user-attachments/assets/a7835f88-9795-4aeb-98d6-ada78cce8246

**Expected Result:**
The error appears: "Only amino acids and CHEMs with BILN codes can get exported to BILN." when the user exports a monomer (amino acid or CHEM) without a BILN alias

As per reqs:

**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

Reproduce the issue from Macromolecules - Flex mode using an amino acid without a BILN alias, then trace the Save as flow when BILN is selected. The work is done when exporting an amino acid or CHEM without a BILN code shows the specified error instead of producing a BILN file.

Written by the indexing model from the issue text.