Exported string chosen for circular backbone is not first alphabetically

Found while testing #3541

Note: after fixing this issue, requirement 2.1 of the Export 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 HELM string from the clipboard: `PEPTIDE1{C.[Tza].A.A.[Tza]}|PEPTIDE2{[aIle].[Abu].[dE].[Bip]}$PEPTIDE2,PEPTIDE1,3:R3-1:R3|PEPTIDE1,PEPTIDE1,1:R1-5:R2|PEPTIDE2,PEPTIDE2,1:R1-4:R2$$$V2.0`
3. Click `Open as New` or `Add to Canvas` button
4. Click `Save as` button
5. Select BILN as the file format

**Actual Result:**
Strings chosen for circular backbones are not first alphabetically: `C-Tza-A-A-Tza` and `aIle-Abu-dE-Bip`

**Expected Result:**
Strings chosen for the circular backbones are first alphabetically.
`A-A-Tza-C-Tza` is the first alphabetically (two 'a's), and `Abu-dE-Bib-alle` is the first alphabetically ('ab'). So the BILN string is `A(1,2)-A-Tza-C(2,3)-Tza(1,1).Abu(3,1)-dE(2,3)-Bip-alle(3,2)`

As per Req:

**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

Start by reproducing the issue in Macromolecules Flex mode with the provided HELM string, then export it as BILN and compare the circular-backbone ordering with the expected string. Re-test requirement 2.1 and the examples in issue #3541; done means both circular backbones use their alphabetically first representation.

Written by the indexing model from the issue text.