tests: validate WorkspaceClient.dbutils lazy-property behavior

Add four tests covering the cached_property contract:

- ``dbutils`` is a ``functools.cached_property`` descriptor on
  ``WorkspaceClient``.
- ``WorkspaceClient.__init__`` does not invoke ``_make_dbutils``.
- The first ``ws.dbutils`` read invokes ``_make_dbutils`` exactly once;
  subsequent reads return the cached value without re-invoking.
- Constructing ``WorkspaceClient`` on a faked Spark Connect runtime
  (whose ``dbruntime`` raises ``CONTEXT_UNAVAILABLE_FOR_REMOTE_CLIENT``
  on any namespace materialization) succeeds without importing
  ``databricks.sdk.runtime`` at all — the durable sidestep of
  databricks/dbt-databricks#1252.

Author: Divyansh-db

tests/test_client.py

@@ -1,3 +1,6 @@
+import functools
+import sys
+import types
 from unittest.mock import create_autospec
 
 import pytest
@@ -15,3 +18,65 @@ def test_autospec_fails_on_setting_unknown_property():
     w = create_autospec(WorkspaceClient, spec_set=True)
     with pytest.raises(AttributeError):
         w.bar = 1
+
+
+def test_dbutils_is_a_cached_property():
+    """``dbutils`` is a ``functools.cached_property`` so consumers that never read it
+    pay no build cost — and, on Spark Connect runtimes, never touch the legacy
+    ``SparkContext`` path that ``databricks.sdk.runtime`` materializes on import."""
+    descriptor = WorkspaceClient.__dict__["dbutils"]
+    assert isinstance(descriptor, functools.cached_property)
+
+
+def test_workspace_client_init_does_not_build_dbutils(config, mocker):
+    """Constructing a ``WorkspaceClient`` must not invoke ``_make_dbutils``."""
+    spy = mocker.patch("databricks.sdk._make_dbutils")
+
+    WorkspaceClient(config=config)
+
+    spy.assert_not_called()
+
+
+def test_dbutils_first_access_builds_exactly_once(config, mocker):
+    """First read of ``.dbutils`` calls ``_make_dbutils`` once; subsequent reads
+    return the cached value without re-invoking."""
+    sentinel = object()
+    spy = mocker.patch("databricks.sdk._make_dbutils", return_value=sentinel)
+    ws = WorkspaceClient(config=config)
+
+    first = ws.dbutils
+    assert spy.call_count == 1
+    assert first is sentinel
+
+    second = ws.dbutils
+    assert spy.call_count == 1  # still 1 — cached_property short-circuits via __dict__
+    assert second is sentinel
+
+
+def test_workspace_client_constructs_on_spark_connect_without_touching_runtime(monkeypatch, config):
+    """End-to-end Layer 2 win: with the lazy property, ``WorkspaceClient(config=...)``
+    on a Spark Connect cluster succeeds without ever importing
+    ``databricks.sdk.runtime`` — so the legacy ``SparkContext`` materialization that
+    raises ``CONTEXT_UNAVAILABLE_FOR_REMOTE_CLIENT`` is never even attempted.
+
+    Faked ``dbruntime`` raises on any namespace materialization; if anything during
+    construction triggered ``databricks.sdk.runtime``'s import, this test would crash.
+    """
+
+    class _Initializer:
+        @staticmethod
+        def getOrCreate():
+            raise RuntimeError(
+                "[CONTEXT_UNAVAILABLE_FOR_REMOTE_CLIENT] Calls to SparkContext are not "
+                "supported on a Spark Connect cluster."
+            )
+
+    fake = types.ModuleType("dbruntime")
+    fake.UserNamespaceInitializer = _Initializer
+    monkeypatch.setitem(sys.modules, "dbruntime", fake)
+    monkeypatch.delitem(sys.modules, "databricks.sdk.runtime", raising=False)
+
+    ws = WorkspaceClient(config=config)
+
+    assert ws is not None
+    assert "databricks.sdk.runtime" not in sys.modules