Fix flaky codesnippetfromselectedcells.cy.ts 2-cells race

The 'test 2 cells' case clicks the toolbar add-cell button and then
calls populateCells, which snapshots cy.get('.jp-Cell') once. If
JupyterLab has not yet mounted the second cell, only one cell gets
typed content. The subsequent shift-click multi-selection then
includes an empty cell.

save-as-snippet's isEnabled requires getSelectedCellContents().length
> 0 (packages/code-snippet/src/index.ts:90-112), so when an empty
cell is part of the active selection the menu item renders disabled.
The retry helper only checks presence, so it exits the loop, the
click fires on a disabled item and no-ops, and the test times out
waiting for .elyra-metadataEditor.

Wait for the .jp-Cell count to grow after the add-cell click,
mirroring the formArrayAdd pattern from #3389.

Signed-off-by: Luciano Resende <lresende@apple.com>

Author: lresende

cypress/tests/codesnippetfromselectedcells.cy.ts

@@ -66,10 +66,16 @@ describe('Code snippet from cells tests', () => {
   });
 
   it('test 2 cells', () => {
-    // Create new cells
-    cy.get(
-      '.jp-NotebookPanel-toolbar > div:nth-child(2) > jp-button:nth-child(1)'
-    ).click();
+    // Create a new cell and wait for it to mount before populating.
+    // populateCells snapshots cy.get('.jp-Cell') once; without this wait
+    // the second cell may not exist yet, leaving it empty so save-as-snippet
+    // renders disabled and the .elyra-metadataEditor wait times out.
+    cy.get('.jp-Cell').then(($before) => {
+      cy.get(
+        '.jp-NotebookPanel-toolbar > div:nth-child(2) > jp-button:nth-child(1)'
+      ).click();
+      cy.get('.jp-Cell').should('have.length.greaterThan', $before.length);
+    });
 
     waitForKernelIdle();