feat(api): (1.2/n) datasets.iterrorws pagination api updates (#1656)

# What does this PR do?
- as title
- uses "cursor" pagination scheme for iterrows

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

## Test Plan
<img width="1226" alt="image"
src="https://github.com/user-attachments/assets/3220eaac-7117-4d0a-b344-2bbb77a22065"
/>


[//]: # (## 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}/rows:
+  /v1/datasets/{dataset_id}/append-rows:
     post:
       responses:
         '200':
@@ -1465,7 +1465,7 @@ paths:
           content:
             application/json:
               schema:
-                $ref: '#/components/schemas/PaginatedRowsResult'
+                $ref: '#/components/schemas/IterrowsResponse'
         '400':
           $ref: '#/components/responses/BadRequest400'
         '429':
@@ -1479,7 +1479,7 @@ paths:
       tags:
         - DatasetIO
       description: >-
-        Get a paginated list of rows from a dataset.
+        Get a paginated list of rows from a dataset. Uses cursor-based pagination.
       parameters:
         - name: dataset_id
           in: path
@@ -1488,25 +1488,19 @@ paths:
           required: true
           schema:
             type: string
-        - name: rows_in_page
-          in: query
-          description: The number of rows to get per page.
-          required: true
-          schema:
-            type: integer
-        - name: page_token
-          in: query
-          description: The token to get the next page of rows.
-          required: false
-          schema:
-            type: string
-        - name: filter_condition
+        - name: start_index
           in: query
           description: >-
-            (Optional) A condition to filter the rows by.
+            Index into dataset for the first row to get. Get all rows if None.
           required: false
           schema:
-            type: string
+            type: integer
+        - name: limit
+          in: query
+          description: The number of rows to get per page.
+          required: false
+          schema:
+            type: integer
   /v1/eval/benchmarks/{benchmark_id}/jobs/{job_id}:
     get:
       responses:
@@ -5547,10 +5541,10 @@ components:
       required:
         - content
       title: ToolInvocationResult
-    PaginatedRowsResult:
+    IterrowsResponse:
       type: object
       properties:
-        rows:
+        data:
           type: array
           items:
             type: object
@@ -5563,17 +5557,15 @@ components:
                 - type: array
                 - type: object
           description: The rows in the current page.
-        total_count:
+        next_index:
           type: integer
-          description: The total number of rows in the dataset.
-        next_page_token:
-          type: string
-          description: The token to get the next page of rows.
+          description: >-
+            Index into dataset for the first row in the next page. None if there are
+            no more rows.
       additionalProperties: false
       required:
-        - rows
-        - total_count
-      title: PaginatedRowsResult
+        - data
+      title: IterrowsResponse
       description: A paginated list of rows from a dataset.
     ListAgentSessionsResponse:
       type: object