Fix session_list to return tuple of (session_references, session_list)

The method previously returned a single list, causing a ValueError when
unpacking into two variables (sessrefs, sesslist = D.session_list()).
Now returns a tuple matching the MATLAB convention.

https://claude.ai/code/session_011dfeoCuRP6gCP6R9FYk2rH

Author: claude

src/ndi/dataset/_dataset.py

@@ -224,33 +224,38 @@ def open_session(self, session_id: str) -> Any | None:
 
         return session
 
-    def session_list(self) -> list[dict[str, Any]]:
+    def session_list(self) -> tuple[list[str], list[dict[str, Any]]]:
         """
         List all sessions in this dataset.
 
         Returns:
-            List of dicts with keys:
-                - session_id: Session identifier
-                - session_reference: Session reference name
-                - is_linked: True if linked, False if ingested
-                - document_id: ID of the session_in_a_dataset document
+            A tuple of (session_references, session_list) where:
+                - session_references: List of session reference strings
+                - session_list: List of dicts with keys:
+                    - session_id: Session identifier
+                    - session_reference: Session reference name
+                    - is_linked: True if linked, False if ingested
+                    - document_id: ID of the session_in_a_dataset document
         """
         q = Query("").isa("session_in_a_dataset")
         docs = self._session.database_search(q)
 
-        result = []
+        session_references = []
+        session_list = []
         for doc in docs:
             props = doc.document_properties.get("session_in_a_dataset", {})
-            result.append(
+            ref = props.get("session_reference", "")
+            session_references.append(ref)
+            session_list.append(
                 {
                     "session_id": props.get("session_id", ""),
-                    "session_reference": props.get("session_reference", ""),
+                    "session_reference": ref,
                     "is_linked": bool(props.get("is_linked", False)),
                     "document_id": doc.id,
                 }
             )
 
-        return result
+        return session_references, session_list
 
     # =========================================================================
     # Database Operations (delegated to internal session)