feat: reference implementation for files API (#2330)

# What does this PR do?
TSIA
Added Files provider to the fireworks template. Might want to add to all
templates as a follow-up.

## Test Plan
llama-stack pytest tests/unit/files/test_files.py

llama-stack llama stack build --template fireworks --image-type conda
--run
LLAMA_STACK_CONFIG=http://localhost:8321 pytest -s -v
tests/integration/files/

tests/integration/files/test_files.py

@@ -0,0 +1,51 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from io import BytesIO
+
+
+def test_openai_client_basic_operations(openai_client):
+    """Test basic file operations through OpenAI client."""
+    client = openai_client
+
+    test_content = b"files test content"
+
+    try:
+        # Upload file using OpenAI client
+        with BytesIO(test_content) as file_buffer:
+            file_buffer.name = "openai_test.txt"
+            uploaded_file = client.files.create(file=file_buffer, purpose="assistants")
+
+        # Verify basic response structure
+        assert uploaded_file.id.startswith("file-")
+        assert hasattr(uploaded_file, "filename")
+
+        # List files
+        files_list = client.files.list()
+        file_ids = [f.id for f in files_list.data]
+        assert uploaded_file.id in file_ids
+
+        # Retrieve file info
+        retrieved_file = client.files.retrieve(uploaded_file.id)
+        assert retrieved_file.id == uploaded_file.id
+
+        # Retrieve file content - OpenAI client returns httpx Response object
+        content_response = client.files.content(uploaded_file.id)
+        # The response is an httpx Response object with .content attribute containing bytes
+        content = content_response.content
+        assert content == test_content
+
+        # Delete file
+        delete_response = client.files.delete(uploaded_file.id)
+        assert delete_response.deleted is True
+
+    except Exception as e:
+        # Cleanup in case of failure
+        try:
+            client.files.delete(uploaded_file.id)
+        except Exception:
+            pass
+        raise e