fix: correctly passing the prompt provided by the user (#3252)

Closes CORE-207

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

backend/api/quivr_api/modules/rag_service/rag_service.py

@@ -263,6 +263,9 @@ class RAGService:
 
         llm = self.get_llm(retrieval_config)
 
+        if self.prompt:
+            retrieval_config.prompt = self.prompt.content
+
         # Get model metadata
         model_metadata = await self.model_service.get_model(self.brain.name)