Feat: File uploads for future conversations on SDK API (#13378)

### What problem does this PR solve?

This PR aims to:

1. Enable file uploads for the public API, similarly to what
/document/upload_info accomplishes for the frontend;
2. Enable files sent to the /chat/:chat_id/completions endpoint to be
used within the conversation.
We classify the first item as a new future, while classifying the second
one as a bug fix.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)

*The work related to this PR was co-authored by*

[Bruno Ferreira](https://github.com/brunopferreira): Custom Solutions
Manager @ [Orbcom](https://orbcom.pt/)
[Pedro Ferreira](https://github.com/sirj0k3r): Lead Software Developer @
[Orbcom](https://orbcom.pt/)
[Pedro Cardoso](https://github.com/pedromiguel4560): Associate Software
Developer @ [Orbcom](https://orbcom.pt/)

*This PR replaces #13248*

---------

Co-authored-by: Pedro Cardoso <pedrocardoso@orbcom.pt>
Co-authored-by: Pedro Ferreira <pedroferreira@orbcom.pt>

api/db/services/conversation_service.py

@@ -158,6 +158,11 @@ async def async_completion(tenant_id, chat_id, question, name="New session", ses
         "role": "user",
         "id": str(uuid4())
     }
+
+    # Propagate runtime attachments so downstream chat flow can resolve file content.
+    if isinstance(kwargs.get("files"), list) and kwargs["files"]:
+        question["files"] = kwargs["files"]
+
     conv.message.append(question)
     for m in conv.message:
         if m["role"] == "system":