Fix content-categoriser output JSON structure

[gvzdv]

This PR ensures that the output of the content-categoriser preprocessor is in accordance with the schema.
After refactoring LLM-based components, LLM produces the output formatted as {'category': '_category_name_'}.

I mistakenly left in the code the additional structure that was assigning this key-value pair again, making the output look like '{category: '{'category': '_category_name_'}} causing the preprocessor to fail the validation.

Tested on Unicorn: now the output is validated successfully.

---

Required Information

• I referenced the issue addressed in this PR.
• I described the changes made and how these address the issue.
• I described how I tested these changes.

Coding/Commit Requirements

• I followed applicable coding standards where appropriate (e.g., PEP8)
• I have not committed any models or other large files.

New Component Checklist (mandatory for new microservices)

• I added an entry to docker-compose.yml and build.yml.
• I created A CI workflow under .github/workflows.
• I have created a README.md file that describes what the component does and what it depends on (other microservices, ML models, etc.).

OR

• I have not added a new component in this PR.