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)

llama_stack/apis/datasetio/datasetio.py

@@ -34,7 +34,8 @@ class DatasetIO(Protocol):
     # keeping for aligning with inference/safety, but this is not used
     dataset_store: DatasetStore
 
-    @webmethod(route="/datasets/{dataset_id}/iterrows", method="GET")
+    # TODO(xiyan): there's a flakiness here where setting route to "/datasets/" here will not result in proper routing
+    @webmethod(route="/datasetio/iterrows/{dataset_id:path}", method="GET")
     async def iterrows(
         self,
         dataset_id: str,
@@ -49,5 +50,5 @@ class DatasetIO(Protocol):
         """
         ...
 
-    @webmethod(route="/datasets/{dataset_id}/append-rows", method="POST")
+    @webmethod(route="/datasetio/append-rows/{dataset_id:path}", method="POST")
     async def append_rows(self, dataset_id: str, rows: List[Dict[str, Any]]) -> None: ...