Send a Slack notification on a non-successful iOS E2E workflow

This commit introduces a notification on any case of a `completed` but
unsuccessful workflow (i.e. where the conclusion is not `success`).

**Rationale**: We currently send a notification if the E2E tests run
and fail. Due to runner issues some scheduled nightly runs never get
executed. As we only get notified of failing runs, we assume that 
"No news is good news", but that is not necessarily true.

According to [this GitHub documentation][1]
we should be able to monitor whether the iOS E2E test workflow is
completed, which fits wanting to be informed about _both_ failed tests
and scheduled runs, regardless of the reason.

**How to test**: ~We should be able to execute the workflow using this
PR/branch.~ According to the [documentation][2] this will only run on
the `default` branch:

> This event will only trigger a workflow run if the workflow file
> exists on the default branch.

**Technical details**:

- [Workflow_run][3]
has 3 `action types`: `requested`, `in_progress` and `completed`.
- The `workflow_run` payload is documented [here][4].

**Note**: As I did not delete the original notification in
`ios-end-to-end-tests-nightly.yml`, failed runs will notify us twice
right now. I'd propose to remove that notification _after_ we've
observed this notification to work consistently.

**Caveat**: I did come across 
https://github.com/orgs/community/discussions/21090, but given the
amount of time that has passed, I'm curious to see if that issue has
been resolved in the meantime.

[1]: https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#running-a-workflow-based-on-the-conclusion-of-another-workflow
[2]: https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#workflow_run
[3]: https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=completed#workflow_run
[4]: https://docs.github.com/en/rest/actions/workflow-runs?apiVersion=2026-03-10#get-a-workflow-run

Author: daneov

.github/workflows/ios-end-to-end-tests-nightly-notification.yml

@@ -0,0 +1,28 @@
+---
+name: iOS end-to-end nightly tests monitor
+on:
+  # https://docs.github.com/en/webhooks/webhook-events-and-payloads#workflow_run
+  workflow_run:
+    workflows: ["iOS end-to-end nightly tests"]
+    types:
+      - completed
+
+permissions: {}
+
+jobs:
+  notify-on-failed-workflow:
+    # Payload of workflow_run:
+    # https://docs.github.com/en/rest/actions/workflow-runs?apiVersion=2026-03-10#get-a-workflow-run
+    if: "${{ github.event.workflow_run.conclusion != 'successful' }}"
+    name: Notify team on nightly E2E failure
+    runs-on: [ubuntu-latest]
+    timeout-minutes: 5
+    steps:
+      - name: Send custom event details to a Slack workflow
+        uses: slackapi/slack-github-action@v2.1.1
+        with:
+          webhook: ${{ secrets.IOS_SLACK_WEBHOOK_URL }}
+          webhook-type: webhook-trigger
+          # workflow_run.html_url returns the full url of the workflow we're responding too.
+          payload: |
+            run_url: "${{ github.event.workflow_run.html_url }}"