ci: Reduce unit tests concurrency from 2 to 1

Our tests are unstable and flaky when running with pytest numprocesses > 1, especially on Windows and macOS. This change reduces concurrency to 1 to determine if the instability is caused by parallel execution or if there are underlying issues in the code/tests.

If we want higher concurrency in the future, we must properly address the root cause of test instability rather than masking it with parallel execution.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: vdusek

.github/workflows/_tests.yaml

@@ -17,4 +17,4 @@ jobs:
       operating_systems: '["ubuntu-latest", "windows-latest", "macos-latest"]'
       python_version_for_codecov: "3.14"
       operating_system_for_codecov: ubuntu-latest
-      tests_concurrency: "2"
+      tests_concurrency: "1"