fix: Partition aware source table UI widget swaps from merge to coalesce (#2397)

- Fixes the observed issue of the partition selector button flipping
between Merge and Coalesce depending on which partition view is
selected.
- The original condition for `isPartitionAwareSourceTable` in
`IrisGridTableModel.ts` isn't always accurate as `table.isUncoalesced`
is only true when no filters have been applied yet. Once a filter is
applied, the table is no longer considered uncoalesced but the source
table should still have the same property as before.
- Test snippet is linked on the ticket:
[DH-19015](https://deephaven.atlassian.net/browse/DH-19015)

[DH-19015]:
https://deephaven.atlassian.net/browse/DH-19015?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: ericlln

packages/iris-grid/src/IrisGridProxyModel.ts

@@ -442,6 +442,13 @@ class IrisGridProxyModel extends IrisGridModel implements PartitionedGridModel {
       : undefined;
   }
 
+  get isPartitionAwareSourceTable(): boolean {
+    return (
+      isPartitionedGridModelProvider(this.originalModel) &&
+      this.originalModel.isPartitionAwareSourceTable
+    );
+  }
+
   get formatter(): Formatter {
     return this.originalModel.formatter;
   }

packages/iris-grid/src/IrisGridTableModel.ts

@@ -37,6 +37,9 @@ class IrisGridTableModel
 
   initialFilters: DhType.FilterCondition[] = [];
 
+  // The initial value for table.isUncoalesced on the source table before any transformations (e.g. filter, select)
+  wasUncoalesced: boolean;
+
   /**
    * @param dh JSAPI instance
    * @param table Iris data table to be used in the model
@@ -53,6 +56,7 @@ class IrisGridTableModel
     this.customColumnList = [];
     this.formatColumnList = [];
     this.initialFilters = table.filter;
+    this.wasUncoalesced = this.isPartitionRequired;
   }
 
   get isExportAvailable(): boolean {
@@ -301,7 +305,7 @@ class IrisGridTableModel
   }
 
   get isPartitionAwareSourceTable(): boolean {
-    return this.isPartitionRequired;
+    return this.wasUncoalesced;
   }
 
   isFilterable(columnIndex: ModelIndex): boolean {