fix: DH-18976: Add proper bounds to pendingRow logic (#2457)

Part of DH-18976. Fixes an issue where the pendingRow check didn't
account for totals rows, causing aggregations to be incorrectly flagged
as pending in the color logic.

Author: gzh2003

packages/iris-grid/src/IrisGridTableModelTemplate.ts

@@ -1034,7 +1034,12 @@ class IrisGridTableModelTemplate<
    * @returns The row within the pending input rows if it's a pending row, null otherwise
    */
   pendingRow(y: ModelIndex): ModelIndex | null {
+    if (!this.isEditable) {
+      return null;
+    }
+
     const pendingRow = y - this.floatingTopRowCount - this.table.size;
+
     if (pendingRow >= 0) {
       return pendingRow;
     }