fix: Do not show Group column for tree-tables

- If a table has no grouped columns or only one grouped column, no point in adding an extra Group column for that situation as it is just duplicate information
- Fixes deephaven#1831
- Added unit tests, and tested with the following snippet:
```python
from deephaven import read_csv, agg
from deephaven.constants import NULL_INT
from deephaven import empty_table

_source = empty_table(100).update_view(["ID = i", "Parent = i == 0 ? NULL_INT : (int)(i / 4)"])
_insurance = read_csv("https://media.githubusercontent.com/media/deephaven/examples/main/Insurance/csv/insurance.csv")

agg_list = [agg.avg(cols=["bmi", "expenses"])]
small_by_list = ["region"]
by_list = ["region", "age"]

no_group_tree = _source.tree(id_col="ID", parent_col="Parent")
no_group_agg = insurance.rollup(aggs=agg_list, by=small_by_list, include_constituents=True)

group_rollup_no_agg = insurance.rollup(aggs=[], by=by_list, include_constituents=True)
group_rollup_agg = insurance.rollup(aggs=agg_list, by=by_list, include_constituents=True)
```

__mocks__/dh-core.js

@@ -841,6 +841,16 @@ class Table extends DeephavenObject {
   }
 }
 
+// TreeTable and Table are different in actual implementation, but should be okay for the mock
+class TreeTable extends Table {
+  constructor(props) {
+    super(props);
+
+    const { groupedColumns = [] } = props;
+    this.groupedColumns = groupedColumns;
+  }
+}
+
 Table.EVENT_CUSTOMCOLUMNSCHANGED = 'customcolumnschanged';
 Table.EVENT_FILTERCHANGED = 'filterchanged';
 Table.EVENT_ROWADDED = 'rowadded';
@@ -1991,8 +2001,7 @@ const dh = {
   TotalsTableConfig: TotalsTableConfig,
   TableViewportSubscription,
   TableSubscription,
-  // TreeTable and Table are different in actual implementation, but should be okay for the mock
-  TreeTable: Table,
+  TreeTable: TreeTable,
   Column: Column,
   RangeSet,
   Row: Row,

packages/iris-grid/src/IrisGridTestUtils.ts

@@ -110,11 +110,17 @@ class IrisGridTestUtils {
 
   makeTreeTable(
     columns = this.makeColumns(),
+    groupedColumns: Column[] = [],
     size = 1000000000,
     sort = []
   ): TreeTable {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    const table = new (this.dh as any).TreeTable({ columns, size, sort });
+    const table = new (this.dh as any).TreeTable({
+      columns,
+      groupedColumns,
+      size,
+      sort,
+    });
     table.copy = jest.fn(() => Promise.resolve(table));
     return table;
   }

packages/iris-grid/src/IrisGridTreeTableModel.test.ts

@@ -0,0 +1,29 @@
+import dh from '@deephaven/jsapi-shim';
+import IrisGridTestUtils from './IrisGridTestUtils';
+import IrisGridTreeTableModel from './IrisGridTreeTableModel';
+
+const irisGridTestUtils = new IrisGridTestUtils(dh);
+
+describe('IrisGridTreeTableModel', () => {
+  const expectedVirtualColumn = expect.objectContaining({
+    name: '__DH_UI_GROUP__',
+    displayName: 'Group',
+    isProxy: true,
+  });
+  const columns = irisGridTestUtils.makeColumns();
+
+  test.each([
+    [0, columns, columns],
+    [1, columns, columns],
+    [2, columns, [expectedVirtualColumn, ...columns]],
+    [columns.length, columns, [expectedVirtualColumn, ...columns]],
+  ])(
+    `create virtual columns with group length %`,
+    (groupedLength, allColumns, expected) => {
+      const groupedColumns = allColumns.slice(0, groupedLength);
+      const table = irisGridTestUtils.makeTreeTable(allColumns, groupedColumns);
+      const model = new IrisGridTreeTableModel(dh, table);
+      expect(model.columns).toEqual(expected);
+    }
+  );
+});

packages/iris-grid/src/IrisGridTreeTableModel.ts

@@ -41,30 +41,33 @@ class IrisGridTreeTableModel extends IrisGridTableModelTemplate<
     inputTable: InputTable | null = null
   ) {
     super(dh, table, formatter, inputTable);
-    this.virtualColumns = [
-      {
-        name: '__DH_UI_GROUP__',
-        displayName: 'Group',
-        type: TableUtils.dataType.STRING,
-        constituentType: TableUtils.dataType.STRING,
-        isPartitionColumn: false,
-        isSortable: false,
-        isProxy: true,
-        description: 'Key column',
-        filter: () => {
-          throw new Error('Filter not implemented for virtual column');
-        },
-        sort: () => {
-          throw new Error('Sort not implemented virtual column');
-        },
-        formatColor: () => {
-          throw new Error('Color not implemented for virtual column');
-        },
-        formatRowColor: () => {
-          throw new Error('Color not implemented for virtual column');
-        },
-      },
-    ];
+    this.virtualColumns =
+      table.groupedColumns.length > 1
+        ? [
+            {
+              name: '__DH_UI_GROUP__',
+              displayName: 'Group',
+              type: TableUtils.dataType.STRING,
+              constituentType: TableUtils.dataType.STRING,
+              isPartitionColumn: false,
+              isSortable: false,
+              isProxy: true,
+              description: 'Key column',
+              filter: () => {
+                throw new Error('Filter not implemented for virtual column');
+              },
+              sort: () => {
+                throw new Error('Sort not implemented virtual column');
+              },
+              formatColor: () => {
+                throw new Error('Color not implemented for virtual column');
+              },
+              formatRowColor: () => {
+                throw new Error('Color not implemented for virtual column');
+              },
+            },
+          ]
+        : [];
   }
 
   applyBufferedViewport(