Fix CI: reduce xdist workers to 2 to prevent Spark OOM

4 workers still caused OOM on GHA runners (7GB RAM). Reducing to 2
to give Spark tests enough memory headroom.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: eakmanrq <6326532+eakmanrq@users.noreply.github.com>

Author: eakmanrq

.github/workflows/pr.yaml

@@ -76,7 +76,7 @@ jobs:
       matrix:
         python-version: ['3.9', '3.10', '3.11', '3.12', '3.13']
     env:
-      PYTEST_XDIST_AUTO_NUM_WORKERS: 4
+      PYTEST_XDIST_AUTO_NUM_WORKERS: 2
       UV: '1'
     steps:
       - uses: actions/checkout@v5