Guard __enter__ against re-entry on closed client

Add _check_closed() call at the top of __enter__ so that attempting
to use a closed client as a context manager raises RuntimeError
instead of silently creating a new session.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tpellissier

src/PowerPlatform/Dataverse/client.py

@@ -145,7 +145,10 @@ def __enter__(self) -> DataverseClient:
 
         :return: The client instance.
         :rtype: DataverseClient
+
+        :raises RuntimeError: If the client has been closed.
         """
+        self._check_closed()
         if self._session is None:
             self._session = requests.Session()
         return self

tests/unit/test_context_manager.py

@@ -224,6 +224,11 @@ def test_flush_cache_raises_after_close(self):
         with self.assertRaises(RuntimeError):
             self.client.flush_cache("picklist")
 
+    def test_enter_raises_after_close(self):
+        """__enter__ should raise RuntimeError on a closed client."""
+        with self.assertRaises(RuntimeError):
+            self.client.__enter__()
+
     def test_error_message_is_clear(self):
         """The RuntimeError message should clearly state the client is closed."""
         with self.assertRaises(RuntimeError) as ctx: