fix: satisfy strict null checks in new split-view tests

Use proper guards before indexing getAllByTestId results. Store the array
in a variable first and assert the element is defined before using it, to
satisfy noUncheckedIndexedAccess strictness.

client/src/pages/__tests__/Workspace.test.tsx

@@ -664,8 +664,10 @@ describe("split terminal view", () => {
     const { unmount } = await renderWorkspace();
     fireEvent.click(screen.getByRole("button", { name: /2.*pane/i }));
     await settle();
-    const select = screen.getAllByTestId("pane-lane-select")[1];
-    fireEvent.change(select, { target: { value: String(lanesToReturn[1]!.id) } });
+    const selects = screen.getAllByTestId("pane-lane-select");
+    const select = selects[1];
+    expect(select).toBeDefined();
+    fireEvent.change(select!, { target: { value: String(lanesToReturn[1]!.id) } });
     await settle();
     unmount();
 
@@ -676,10 +678,9 @@ describe("split terminal view", () => {
     );
 
     await renderWorkspace();
-    expect(screen.getAllByTestId("pane-lane-select")).toHaveLength(2);
-    expect((screen.getAllByTestId("pane-lane-select")[1] as HTMLSelectElement).value).toBe(
-      String(lanesToReturn[1]!.id)
-    );
+    const persistedSelects = screen.getAllByTestId("pane-lane-select");
+    expect(persistedSelects).toHaveLength(2);
+    expect((persistedSelects[1] as HTMLSelectElement).value).toBe(String(lanesToReturn[1]!.id));
   });
 
   it("falls back to unselected when a persisted lane id no longer exists", async () => {