Show actual number of selected hosts (#46334)

**Related issue:** Resolves #40502

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.


## Testing

- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Fixed "select all matching hosts" to display the actual total count
instead of showing an estimate like "50+" in table headers and delete
confirmation dialogs.

* **Tests**
* Updated test cases to reflect accurate host count display behavior
when selecting all matching hosts.

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frontend/components/TableContainer/DataTable/DataTable.tsx

@@ -50,6 +50,7 @@ interface IDataTableProps {
   isAllPagesSelected: boolean; // TODO: make dependent on showMarkAllPages
   toggleAllPagesSelected?: any; // TODO: an event type and make it dependent on showMarkAllPages
   resultsTitle?: string;
+  totalCount?: number;
   defaultPageSize: number;
   defaultPageIndex?: number;
   defaultSelectedRows?: Record<string, boolean>;
@@ -109,6 +110,7 @@ const DataTable = ({
   isAllPagesSelected,
   toggleAllPagesSelected,
   resultsTitle = "results",
+  totalCount,
   defaultPageSize,
   defaultPageIndex,
   defaultSelectedRows = {},
@@ -452,8 +454,10 @@ const DataTable = ({
     return (
       <p>
         <span>
-          {selectedCount}
-          {isAllPagesSelected && "+"}
+          {isAllPagesSelected && totalCount !== undefined
+            ? totalCount
+            : selectedCount}
+          {isAllPagesSelected && totalCount === undefined && "+"}
         </span>{" "}
         selected
       </p>