feat(dataset api): (1.5/n) fix dataset registeration (#1659)

# What does this PR do?

- fix dataset registeration & iterrows
> NOTE: the URL endpoint is changed to datasetio due to flaky path
routing

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
```
LLAMA_STACK_CONFIG=fireworks pytest -v tests/integration/datasets/test_datasets.py
```
<img width="854" alt="image"
src="https://github.com/user-attachments/assets/0168b352-1c5a-48d1-8e9a-93141d418e54"
/>


[//]: # (## Documentation)

tests/integration/datasets/test_datasets.py

@@ -0,0 +1,95 @@
+# 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.
+
+
+import base64
+import mimetypes
+import os
+
+import pytest
+
+# How to run this test:
+#
+# LLAMA_STACK_CONFIG="template-name" pytest -v tests/integration/datasets
+
+
+def data_url_from_file(file_path: str) -> str:
+    if not os.path.exists(file_path):
+        raise FileNotFoundError(f"File not found: {file_path}")
+
+    with open(file_path, "rb") as file:
+        file_content = file.read()
+
+    base64_content = base64.b64encode(file_content).decode("utf-8")
+    mime_type, _ = mimetypes.guess_type(file_path)
+
+    data_url = f"data:{mime_type};base64,{base64_content}"
+
+    return data_url
+
+
+@pytest.mark.parametrize(
+    "purpose, source, provider_id, limit",
+    [
+        (
+            "eval/messages-answer",
+            {
+                "type": "uri",
+                "uri": "huggingface://datasets/llamastack/simpleqa?split=train",
+            },
+            "huggingface",
+            10,
+        ),
+        (
+            "eval/messages-answer",
+            {
+                "type": "rows",
+                "rows": [
+                    {
+                        "messages": [{"role": "user", "content": "Hello, world!"}],
+                        "answer": "Hello, world!",
+                    },
+                    {
+                        "messages": [
+                            {
+                                "role": "user",
+                                "content": "What is the capital of France?",
+                            }
+                        ],
+                        "answer": "Paris",
+                    },
+                ],
+            },
+            "localfs",
+            2,
+        ),
+        (
+            "eval/messages-answer",
+            {
+                "type": "uri",
+                "uri": data_url_from_file(os.path.join(os.path.dirname(__file__), "test_dataset.csv")),
+            },
+            "localfs",
+            5,
+        ),
+    ],
+)
+def test_register_and_iterrows(llama_stack_client, purpose, source, provider_id, limit):
+    dataset = llama_stack_client.datasets.register(
+        purpose=purpose,
+        source=source,
+    )
+    assert dataset.identifier is not None
+    assert dataset.provider_id == provider_id
+    iterrow_response = llama_stack_client.datasets.iterrows(dataset.identifier, limit=limit)
+    assert len(iterrow_response.data) == limit
+
+    dataset_list = llama_stack_client.datasets.list()
+    assert dataset.identifier in [d.identifier for d in dataset_list]
+
+    llama_stack_client.datasets.unregister(dataset.identifier)
+    dataset_list = llama_stack_client.datasets.list()
+    assert dataset.identifier not in [d.identifier for d in dataset_list]