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)

docs/_static/llama-stack-spec.html

@@ -40,7 +40,7 @@
         }
     ],
     "paths": {
-        "/v1/datasets/{dataset_id}/append-rows": {
+        "/v1/datasetio/append-rows/{dataset_id}": {
             "post": {
                 "responses": {
                     "200": {
@@ -60,7 +60,7 @@
                     }
                 },
                 "tags": [
-                    "Datasets"
+                    "DatasetIO"
                 ],
                 "description": "",
                 "parameters": [
@@ -2177,7 +2177,7 @@
                 }
             }
         },
-        "/v1/datasets/{dataset_id}/iterrows": {
+        "/v1/datasetio/iterrows/{dataset_id}": {
             "get": {
                 "responses": {
                     "200": {
@@ -2204,7 +2204,7 @@
                     }
                 },
                 "tags": [
-                    "Datasets"
+                    "DatasetIO"
                 ],
                 "description": "Get a paginated list of rows from a dataset. Uses cursor-based pagination.",
                 "parameters": [
@@ -10274,7 +10274,7 @@
             "name": "Benchmarks"
         },
         {
-            "name": "Datasets"
+            "name": "DatasetIO"
         },
         {
             "name": "Datasets"
@@ -10342,7 +10342,7 @@
                 "Agents",
                 "BatchInference (Coming Soon)",
                 "Benchmarks",
-                "Datasets",
+                "DatasetIO",
                 "Datasets",
                 "Eval",
                 "Files",