fix: return 200 on duplicate save instead of crash code "500"

[Aditya-Sarna]

Problem

• Re-saving the same circuit (no changes) hit duplicate detection but returned 500
• Response used input serializer .data → get_lti_id() got a dict instead of a model → AttributeError
• Frontend showed "Saved Successfully" but save actually failed

Fix

• Serialize the existing DB record: StateSaveSerializer(queryset).data
• Use validated_data for name/description updates
• Return 200 with "duplicate": true

Testing

• First save → 200, URL gets id=...
• Same circuit saved again → 200 with "duplicate": true (was 500)
• Verified via browser Network tab and POST /api/save curl