Improve injector contract content fetching

[MarineLeM]

Description

In the application, there are two different behaviors when fetching the inject contract content:

1. Fetching inject contract content as a string

• The backend returns the inject contract content as a string, which is then parsed on the frontend.
• Example API endpoint: /api/injector_contracts/search (used when fetching all inject contracts for creating an atomic test).
  request answer :

2. Fetching inject contract content as both a string and a converted JSON property

• The backend provides the inject contract content in two different formats
• Example API endpoint: api/injects/{injectId}
  request answer :
  

Proposed Solution :

The inject contract content should always be returned as a JSON object, avoiding string parsing.

A possible improvement is to declare it as ContractContent in the backend instead of using ObjectNode, ensuring a clear and structured API response.