is_databricks_runtime integration test — implementation plan

See the design.

1. Add tests/is_databricks_runtime_task.py (remote script: driver + UDF checks, raises RuntimeError on failure).
2. Add test_is_databricks_runtime to tests/test_databricks.py: local negative baseline, wheel build, workspace upload, jobs.submit on serverless, result/error assertions, cleanup in finally.
3. Verify: hatch test -k test_is_databricks_runtime, then a one-off inverted-assertion run to confirm the failure path shows the remote trace, then make format and make test.