feat(api): don't return a payload on file delete

This is to stay consistent with other APIs.

This change registers files in API, even though there are still no
providers. Removing tests that require a provider existing for a merged
API to enable it in API layer.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

llama_stack/apis/datatypes.py

@@ -34,6 +34,7 @@ class Api(Enum):
     scoring_functions = "scoring_functions"
     benchmarks = "benchmarks"
     tool_groups = "tool_groups"
+    files = "files"
 
     # built-in API
     inspect = "inspect"

llama_stack/apis/files/files.py

@@ -164,7 +164,7 @@ class Files(Protocol):
         self,
         bucket: str,
         key: str,
-    ) -> FileResponse:
+    ) -> None:
         """
         Delete a file identified by a bucket and key.