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.yaml

@@ -10,7 +10,7 @@ info:
 servers:
   - url: http://any-hosted-llama-stack.com
 paths:
-  /v1/datasets/{dataset_id}/append-rows:
+  /v1/datasetio/append-rows/{dataset_id}:
     post:
       responses:
         '200':
@@ -26,7 +26,7 @@ paths:
         default:
           $ref: '#/components/responses/DefaultError'
       tags:
-        - Datasets
+        - DatasetIO
       description: ''
       parameters:
         - name: dataset_id
@@ -1457,7 +1457,7 @@ paths:
             schema:
               $ref: '#/components/schemas/InvokeToolRequest'
         required: true
-  /v1/datasets/{dataset_id}/iterrows:
+  /v1/datasetio/iterrows/{dataset_id}:
     get:
       responses:
         '200':
@@ -1477,7 +1477,7 @@ paths:
         default:
           $ref: '#/components/responses/DefaultError'
       tags:
-        - Datasets
+        - DatasetIO
       description: >-
         Get a paginated list of rows from a dataset. Uses cursor-based pagination.
       parameters:
@@ -6931,7 +6931,7 @@ tags:
       Agents API for creating and interacting with agentic systems.
   - name: BatchInference (Coming Soon)
   - name: Benchmarks
-  - name: Datasets
+  - name: DatasetIO
   - name: Datasets
   - name: Eval
     x-displayName: >-
@@ -6971,7 +6971,7 @@ x-tagGroups:
       - Agents
       - BatchInference (Coming Soon)
       - Benchmarks
-      - Datasets
+      - DatasetIO
       - Datasets
       - Eval
       - Files