fix(admin-ui): surface custom report delete failures instead of reporting success

[srinath2404]

Description Of Changes

In the Data Map report's Custom Reports popover, deleting a report calls the delete mutation without awaiting or checking its result, and clears the currently-applied report before the request resolves (CustomReportTemplates.tsx handleDeleteReport).

If the DELETE fails, the UI shows no error, the applied report/view is wiped, and a success-looking outcome is presented — yet the report still exists on the server (the list and trigger label only refresh on a successful, tag-invalidating mutation), so it silently reappears on reload.

This PR awaits the mutation, surfaces an error on failure (matching the sibling handleSelection), and only clears the applied report once the delete actually succeeds.

Code Changes

• clients/admin-ui/src/features/common/custom-reports/CustomReportTemplates.tsx: handleDeleteReport now awaits deleteCustomReportMutationTrigger, shows message.error (via getErrorMessage) on failure and returns early, clears the applied report only after a successful delete, and shows a success message.
• clients/admin-ui/cypress/e2e/datamap-report.cy.ts: added a test asserting that a failed delete surfaces an error and keeps the report (the failure path was previously untested).
• changelog/fix-custom-report-delete-error-handling.yaml: added.

Steps to Confirm

1. Data Map report → Custom Reports popover; apply a saved report.
2. Force the delete to fail (e.g. block/500 */api/v1/plus/custom-report/*).
3. Delete the applied report → an error message is shown; the applied view is preserved; the report remains listed.
4. Without the block, delete succeeds → success message and the report is removed.

Pre-Merge Checklist

• Changelog fragment added
• No migrations
• No documentation updates required
• CI pipelines succeeded
• CLA signed

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