ci: cap E2E full suite to 6 parallel jobs to reduce runner starvation

The 12-shard E2E matrix (6 shards × 2 cdk-source) consumes 13 runners
simultaneously on every main push. During merge storms this starves PR
workflows of runners, causing 45–70 minute queues across all CI.

Adding max-parallel: 6 halves peak runner consumption while only
moderately increasing suite duration (~30 min vs ~15 min).

Author: aidandaly24

.github/workflows/e2e-tests-full.yml

@@ -25,6 +25,7 @@ jobs:
     timeout-minutes: 60
     strategy:
       fail-fast: false
+      max-parallel: 6
       matrix:
         cdk-source: [npm, main]
         shard: ['1/6', '2/6', '3/6', '4/6', '5/6', '6/6']