Fail iOS RNTester E2E straight away when the app artifact is missing

**Stack** (iOS RNTester E2E cost, review bottom-up):
1. #59635
2. #59636
3. #59637
4. #59638
5. #59640
6. #59641 ← this PR
7. #59642

## Summary:

Part 6 of the iOS E2E cost stack.

`test_e2e_ios_rntester` only checks that `test_ios_rntester` succeeded, but `test_ios_rntester` uses `continue-on-error`, so it reports success even when the leg that uploads `RNTesterApp-NewArch-<flavor>` failed (the upload is its last step). In run 40709683649 the Release app was never uploaded and the E2E job still started, and retried, three times.

Download the app as the first step after checkout, and treat a missing app as a non-retryable failure with an error that points to the build job, instead of an incomplete run that triggers the workflow retries.

Gating in `test-all.yml` on the artifact itself would need `actions: read` to list artifacts, which the reusable workflow can't get under Test All's `contents: read`.

## Changelog:

[INTERNAL] [CHANGED] - Fail iOS RNTester E2E straight away when the app artifact is missing

## Test Plan:

- `actionlint` reports no new issues.
- The missing-artifact path matches run 40709683649: `download-artifact` fails after its 3 attempts, the job is classified `failed`, and the run fails without workflow retries.

Differential Revision: D128581762

Pull Request resolved: #59641

Author: cipolleschi

.github/workflows/e2e-ios-rntester.yml

@@ -52,10 +52,9 @@ jobs:
             SHOULD_RUN=false
           fi
           echo "should-run=$SHOULD_RUN" >> "$GITHUB_OUTPUT"
-      - name: Setup Node.js
-        if: steps.test-state.outputs.should-run == 'true'
-        uses: ./.github/actions/setup-node
+      # First, so that a missing app fails the job before any other setup.
       - name: Download App
+        id: download-app
         if: steps.test-state.outputs.should-run == 'true'
         uses: ./.github/actions/download-artifact
         with:
@@ -64,6 +63,9 @@ jobs:
       - name: Check downloaded folder content
         if: steps.test-state.outputs.should-run == 'true'
         run: ls -lR /tmp/RNTesterBuild
+      - name: Setup Node.js
+        if: steps.test-state.outputs.should-run == 'true'
+        uses: ./.github/actions/setup-node
       - name: Run E2E Tests
         id: run-tests
         if: steps.test-state.outputs.should-run == 'true'
@@ -95,14 +97,19 @@ jobs:
         if: always()
         shell: bash
         # passed: every flow passed.
-        # failed: a flow failed all of its attempts; retrying the job would only
-        #   repeat the same failure.
+        # failed: a flow failed all of its attempts, or the app to test is
+        #   missing; retrying the job would only repeat the same failure.
         # incomplete: the job stopped before running every flow (runner or
         #   setup failure); a retry can pick up the remaining flows.
         run: |
           STATE=/tmp/maestro-ios-state/results.json
           if [[ "${{ steps.test-state.outputs.should-run }}" == "false" || "${{ steps.run-tests.outcome }}" == "success" ]]; then
             RESULT=passed
+          elif [[ "${{ steps.download-app.outcome }}" == "failure" ]]; then
+            # test_ios_rntester uses continue-on-error, so it reports success
+            # even when the build that uploads this app failed.
+            echo "::error::RNTesterApp-NewArch-${{ matrix.flavor }} is missing. Check the test_ios_rntester (${{ matrix.flavor }}, frameworks: false) job."
+            RESULT=failed
           elif [[ -f "$STATE" ]] && jq -e 'any(.flows[]; .status == "failed")' "$STATE" > /dev/null; then
             RESULT=failed
           else