Exported string deviates from best practices when exporting previously imported valid input

Found while testing #3541

**Steps to Reproduce:**

1. Open Macromolecules - Flex mode (clean canvas)
2. Paste the following valid BILN string from the clipboard: `C(1,1)-D-E-A(1,2)`
3. Click `Open as New` or `Add to Canvas` button - the structure is loaded on the canvas
4. Click `Save as` button
5. Select BILN as a file format

**Actual Result:**
The exported string deviates from best practices. Actually, it is the same string that was imported

**Expected Result:**
The exported string adheres to best practices and is `A(1,1)-C-D-E(1,2)`

Please refer to the requirements of the 'Export of BILN' section in #3541

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

Start with the Export of BILN requirements in issue #3541, then reproduce the Macromolecules Flex-mode flow using the provided input `C(1,1)-D-E-A(1,2)`. Trace the BILN export path and verify that the saved output follows the required ordering, producing `A(1,1)-C-D-E(1,2)`.

Written by the indexing model from the issue text.