Assert canvasProvider.name key is absent, not just null

Indexing with ["name"] returns Value::Null both when the key is absent
and when it is present as null. Since the wire contract omits name when
None, assert the key is not present to catch a regression that would
serialize name: null.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: jmoseley

rust/tests/session_test.rs

@@ -2950,7 +2950,10 @@ async fn resume_session_sends_canvas_fields_and_captures_open_canvases() {
         request["params"]["canvasProvider"]["id"],
         "app:builtin:window-1"
     );
-    assert!(request["params"]["canvasProvider"]["name"].is_null());
+    assert!(
+        request["params"]["canvasProvider"].get("name").is_none(),
+        "name should be omitted from the wire when None, not serialized as null"
+    );
     assert_eq!(
         request["params"]["openCanvases"][0]["instanceId"],
         "counter-1"