style(core): Start class docstring summaries on the first line

pydocstyle D212 flagged the two class docstrings added by this change
(_ClientCacheMeta, SageMakerClient) once aws#6294 made the sagemaker-core
codestyle job enforceable. Docstring-only change.

---
X-AI-Prompt: fix the failing sagemaker-core codestyle check on PR 6273
X-AI-Tool: Kiro

Author: jam-jee

sagemaker-core/src/sagemaker/core/utils/utils.py

@@ -327,8 +327,7 @@ def __call__(cls, *args, **kwargs):
 
 
 class _ClientCacheMeta(type):
-    """
-    Metaclass that caches instances per constructor arguments.
+    """Metaclass that caches instances per constructor arguments.
 
     A call with no arguments returns the process-wide default instance, creating it
     from the default credential chain if none exists yet. A call with an explicit
@@ -381,8 +380,7 @@ def _evict(cache, default_key, max_keyed_entries):
 
 
 class SageMakerClient(metaclass=_ClientCacheMeta):
-    """
-    Cached factory for SageMaker boto3 clients.
+    """Cached factory for SageMaker boto3 clients.
 
     Clients are cached per (session, region_name, config). Passing an explicit boto3
     ``session`` always yields clients signed with that session's credentials, even if