ci: shard Cloudflare integration tests (#24726)

Similar to #24676, split Cloudflare integration tests across two CI
jobs. Atm this doesn't get us an overall faster wall time but as we'll
soon be running more and more tests on cloudflare this could quickly
become a bottleneck, getting ahead of this by doing this simple
optimization. We can also always add more shards if it becomes
necessary.

Both shards passed. Compared with a run off develop today:

| Cloudflare | Baseline | Two shards | Faster by |
| --- | --- | --- | --- |
| Test execution (slowest shard) | 5m56s | 3m15s | 2m41s (45%) |
| **All Cloudflare jobs finished, from CI start** | **9m31s** |
**6m19s** | **3m12s** |

Observed timings from one run each.

- Baseline: [CI
run](https://github.com/getsentry/sentry-javascript/actions/runs/36110233929)
· [Sentry
trace](https://sentry.sentry.io/explore/traces/trace/c7cd7471a660956b591fea4f99270e09/?project=5899451)
- This PR: [CI
run](https://github.com/getsentry/sentry-javascript/actions/runs/36114956684)
· [Sentry
trace](https://sentry.sentry.io/explore/traces/trace/f2b405b0238b2a25f431bd23da2f130f/?project=5899451)

Co-authored-by: GPT-6 <codex@openai.com>

Author: nicohrubec

.github/workflows/build.yml

@@ -843,11 +843,15 @@ jobs:
         run: yarn test --shard=${{ matrix.shard }}/3
 
   job_cloudflare_integration_tests:
-    name: Cloudflare Integration Tests
+    name: Cloudflare Integration Tests (${{ matrix.shard }}/2)
     needs: [job_get_metadata, job_build]
     if: needs.job_build.outputs.changed_cloudflare_integration == 'true' || github.event_name != 'pull_request'
     runs-on: ubuntu-24.04
     timeout-minutes: 15
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [1, 2]
     steps:
       - name: Check out current commit (${{ needs.job_get_metadata.outputs.commit_label }})
         uses: actions/checkout@v7
@@ -864,7 +868,7 @@ jobs:
 
       - name: Run integration tests
         working-directory: dev-packages/cloudflare-integration-tests
-        run: yarn test
+        run: yarn test --shard=${{ matrix.shard }}/2
 
   job_bundler_plugin_integration_tests:
     name: Bundler Plugin Integration Tests (Node ${{ matrix.node }})