refactor(api): rename "files" API to "artifacts"

The term "artifacts" better represents the purpose of this API, which
handles outputs generated by API executions, eventually stored objects
that can be of served by any storage interface (file, objects).

This aligns better with the industry convention of 'artifacts' (build
outputs, process results) rather than generic 'files'. 'files' would
be appropriate if the goal was to store and retrieve files purely.

Additionally, in our context, artifact is a better term since it will
handle:

* Data produced by SDG (Synthetic Data Generation) - as input
* Output of a trained model - as output

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/utils/scheduler.py

@@ -37,7 +37,7 @@ JobType: TypeAlias = str
 class JobArtifact(BaseModel):
     type: JobType
     name: str
-    # TODO: uri should be a reference to /files API; revisit when /files is implemented
+    # TODO: uri should be a reference to /artifacts API; revisit when /artifacts is implemented
     uri: str | None = None
     metadata: dict[str, Any]