ci: add global concurrency group for E2E tests

Add a concurrency group to ensure only one E2E test job runs at a time
across all workflow runs (PRs, master pushes, manual dispatches).
Previously, max-parallel only limited parallelism within a single
workflow run, so concurrent workflow runs could exceed the platform's
resource limits. Also increase max-parallel to 2 as a compromise between
stability and performance.

Relates to #785

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

Author: vdusek

.github/workflows/_tests.yaml

@@ -85,11 +85,15 @@ jobs:
         (github.event_name == 'push' && github.ref == 'refs/heads/master')
       }}
 
+    # E2E tests build and run Actors on the platform. The combination of max-parallel 2 with 16 pytest
+    # workers and a global concurrency group is a compromise between stability and performance - it keeps
+    # the platform's resource usage in check while still allowing reasonable test throughput.
+    concurrency:
+      group: e2e-tests
+      cancel-in-progress: false
+
     strategy:
-      # E2E tests build and run Actors on the platform. Limit parallel workflows to 1 to avoid exceeding
-      # the platform's memory limits. A single workflow with 16 pytest workers provides good test
-      # parallelization while staying within platform constraints.
-      max-parallel: 1
+      max-parallel: 2
       matrix:
         os: ["ubuntu-latest"]
         python-version: ["3.10", "3.14"]