Refactor molecule_json_saver into smaller translation units

[Copilot]

Remove-me-section

• Please focus review on the split of molecule_json_saver.cpp into targeted units and the shared float writer helper.

Generic request

• PR name follows the pattern #1234 – issue name
• branch name does not contain '#'
• base branch (master or release/xx) is correct
• PR is linked with the issue
• task status changed to "Code review"
• code follows product standards
• regression tests updated

For release/xx branch

• backmerge to master (or newer release/xx) branch is created

Optional

• unit-tests written
• documentation updated

Backmerge request

• PR name follows the pattern Backmerge: #1234 – issue name
• PR is linked with the issue
• base branch (master or release/xx) is correct
• code contains only backmerge changes

Bump request

• PR name follows the pattern Bump version to ...
• add brackets [ ] for 'skip ci' and put it into the body
• milestone is linked to PR
• all tickets are closed inside the relevant milestone

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The oversized molecule_json_saver.cpp was split to keep responsibilities manageable while preserving behavior.

• Changes

  • Extracted SGroup serialization into molecule_json_saver_sgroups.cpp.
  • Extracted monomer/template serialization into molecule_json_saver_monomers.cpp.
  • Extracted text object serialization into molecule_json_saver_text.cpp.
  • Shared saveNativeFloat helper moved to header for cross-TU use; includes trimmed in new files.

• Example

  • Ambiguous monomer ratio now written inside its object:

    if (tg.ratios[i] >= 0) {
        writer.Key(num_name);
        saveNativeFloat(writer, tg.ratios[i], this->native_precision);
    }
    writer.EndObject();

Original prompt

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This section details on the original issue you should resolve

<issue_title>molecule_json_saver.cpp has 2,432 lines of code, which is greater than 2,000 authorized. Split it into smaller files</issue_title>
<issue_description>Problem:
core/indigo-core/molecule/src/molecule_json_saver.cpp has 2,432 lines of code, which is greater than 2,000 authorized. Split it into smaller files.

Why is this an issue?
A source file that grows too much tends to aggregate too many responsibilities and inevitably becomes harder to understand and therefore to maintain. Above a specific threshold, it is strongly advised to refactor it into smaller pieces of code which focus on well defined tasks. Those smaller files will not only be easier to understand but also probably easier to test.

Problem locations:
core/indigo-core/molecule/src/molecule_json_saver.cpp</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes molecule_json_saver.cpp has 2,432 lines of code, which is greater than 2,000 authorized. Split it into smaller files #3414

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