ci: set E2E max-parallel to 4

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

.github/workflows/_tests.yaml

@@ -93,15 +93,15 @@ jobs:
       cancel-in-progress: false
 
     strategy:
-      max-parallel: 2
+      max-parallel: 4
       matrix:
         os: ["ubuntu-latest"]
         python-version: ["3.10", "3.14"]
 
     runs-on: ${{ matrix.os }}
 
     env:
-      TESTS_CONCURRENCY: "8"
+      TESTS_CONCURRENCY: "16"
 
     steps:
       - name: Checkout repository