Fix pytest-xdist serialization issues in nightly tests

Pin pytest-xdist to version >=3.6.1,<4.0 to resolve ExceptionInfo
serialization errors that were causing test failures in the nightly
integration tests. The error occurred when pytest-xdist tried to
serialize exception information between worker processes.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: rauchy

pyproject.toml

@@ -36,7 +36,7 @@ Documentation = "https://databricks-sdk-py.readthedocs.io"
 dev = [
     "pytest",
     "pytest-cov",
-    "pytest-xdist",
+    "pytest-xdist>=3.6.1,<4.0",
     "pytest-mock",
     "black",
     "pycodestyle",