System should NOT save ambiguous amino acid to 1-letter sequence as `X`

**Steps to Reproduce**
1. Select Macromolecules mode:

2. Switch to Flex layout mode (Click on the “A” button in the top panel to open Macromolecules modes switcher
and select the third option "Switch to flex layout mode":

https://github.com/user-attachments/assets/62006ee9-531f-4eda-ac97-b4ec3b16dbd1

3. Copy the following structure in HELM format:
```
PEPTIDE1{(A,C)}$$$$V2.0
```

4. Click the folder icon "Open..." in the top left corner of the top panel

5. In Open Structure pop-up expand the format dropdown by clicking on preselected "Ket" value:

6. Select "HELM"

5. Using Ctrl-V paste previously copied HELM string into Open Structure pop-up

6. Click on "Open as New" button:

7. Click on "Save as..." button:

8. Expand the "File format:" dropdown by clicking on preselected "Ket" value:

9. Select "Sequence (1-letter code)"

**Actual behavior**
System saves it as `X`

**Expected behavior**
Error message should occur

as per [requirement](https://github.com/epam/ketcher/issues/5556):

> 2.3. If one of the amino acids is a non-standard ambiguous amino acid an error message should appear, with the title "Non-standard amino acid" and text "Non-standard ambiguous amino acids cannot be exported to the selected format".
> > By standard ambiguous amino acids we consider amino acids that we have in the library (B, J, X, and Z).
>
>

**Versions**
- Ketcher Version 3.2.0-rc.1 Build at 2025-03-04; 14:24:35
- Indigo Version 1.30.0-rc.1.0-g9bdedaba9-wasm32-wasm-clang-19.0.0
- Chrome Version 134.0.6998.36 (Official Build) (64-bit)
- Win10

Issue found while testing: https://github.com/epam/indigo/issues/2015