[CRE] mixed-env nightly: Slack-alert #cre-platform-operations on failure

A scheduled sweep has no PR to block, so a failure was only visible via GitHub's
default single-recipient email + the Actions tab. Add a notify-on-failure job
(if: failure(), needs the sweep) that posts a rotating-light alert to the
#cre-platform-operations Slack channel with a link to the run, mirroring the
existing CRE Slack-notify pattern (slack-github-action + QA_SLACK_API_KEY).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: prashantkumar1982

.github/workflows/cre-mixed-env-nightly.yaml

@@ -58,3 +58,48 @@ jobs:
       # develop builds and surfaces non-determinism introduced since then.
       mixed_env_baseline_image: "${{ secrets.QA_AWS_ACCOUNT_NUMBER }}.dkr.ecr.${{ secrets.QA_AWS_REGION }}.amazonaws.com/chainlink:nightly-${{ needs.set-dates.outputs.baseline }}"
       full_matrix: true
+
+  # There is no PR to block on a scheduled run, so alert the CRE team on Slack when
+  # the sweep fails (a failure usually means non-determinism was introduced into
+  # develop between the two nightlies, or a job/infra error).
+  notify-on-failure:
+    name: Notify Slack on failure
+    if: failure()
+    needs: [set-dates, mixed-env-sweep]
+    runs-on: ubuntu-latest
+    environment:
+      name: integration
+      deployment: false
+    steps:
+      - name: Send Slack notification (#cre-platform-operations)
+        uses: slackapi/slack-github-action@485a9d42d3a73031f12ec201c457e2162c45d02d # v2.0.0
+        with:
+          errors: "true"
+          method: chat.postMessage
+          token: ${{ secrets.QA_SLACK_API_KEY }}
+          # If delivery fails, invite the bot to the channel, or replace the name
+          # with the channel ID (Cxxxxxxxx) which chat.postMessage resolves more reliably.
+          payload: |
+            {
+              "channel": "#cre-platform-operations",
+              "text": "CRE Mixed-Env Nightly Sweep failed — possible non-determinism introduced into develop.",
+              "blocks": [
+                {
+                  "type": "section",
+                  "text": {
+                    "type": "mrkdwn",
+                    "text": "*:rotating_light: CRE Mixed-Env Nightly Sweep failed :rotating_light:*"
+                  }
+                },
+                {
+                  "type": "section",
+                  "text": {
+                    "type": "mrkdwn",
+                    "text": "Today's develop nightly `nightly-${{ needs.set-dates.outputs.today }}` vs baseline `nightly-${{ needs.set-dates.outputs.baseline }}`. A failure usually means *non-determinism was introduced into develop* between these nightlies (or a job/infra error). Run: <${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}|${{ github.run_id }}>. Please investigate."
+                  }
+                },
+                {
+                  "type": "divider"
+                }
+              ]
+            }

core/scripts/cre/environment/docs/mixed-env.md

@@ -72,7 +72,7 @@ These lines are **silent** when every node runs the same code, so a single occur
 ## When it runs
 
 - **CI:** automatically on CRE-affecting PRs, in its **own** workflow `.github/workflows/cre-mixed-env-tests.yaml` (kept separate from `cre-system-tests.yaml` so that file stays simple). It runs the OCR3/DON2DON-heavy tests — `Test_CRE_V2_Suite_Bucket_A`, `Test_CRE_V2_Suite_Bucket_B`, and the `Test_CRE_V2_EVM_Read_*` suite — under mixed-env. The PR image and the develop image are both already built (per-PR and nightly), so no extra image builds are added. A dedicated **Check for non-determinism** step scans the live node containers after the suite and fails the job on any marker — kept separate from the auto-quarantined test step so the failure can't be swallowed.
-- **Nightly full sweep:** `.github/workflows/cre-mixed-env-nightly.yaml` calls the same dedicated workflow with `full_matrix: true` — running the *entire* matrix under mixed-env (EVM base + Solana/Aptos/Stellar/sharded/cache-test/multi-gateway variants), comparing today's develop nightly against a previous one to catch non-determinism introduced into develop between nightlies.
+- **Nightly full sweep:** `.github/workflows/cre-mixed-env-nightly.yaml` calls the same dedicated workflow with `full_matrix: true` — running the *entire* matrix under mixed-env (EVM base + Solana/Aptos/Stellar/sharded/cache-test/multi-gateway variants), comparing today's develop nightly against a previous one to catch non-determinism introduced into develop between nightlies. A scheduled run has no PR to block, so on failure it posts a `:rotating_light:` alert to the **#cre-platform-operations** Slack channel (with a link to the run) — that's the signal to watch, not a CI check.
 
 ## Required check & emergency bypass