feat: add __enter__ and __aenter__ for context manager support

Client and AsyncClient had __exit__/__aexit__ and close() methods
but were missing __enter__/__aenter__, so `with Client() as c:` and
`async with AsyncClient() as c:` would raise TypeError.

Closes #532

ollama/_client.py

@@ -117,9 +117,15 @@ class BaseClient(contextlib.AbstractContextManager, contextlib.AbstractAsyncCont
       **kwargs,
     )
 
+  def __enter__(self):
+    return self
+
   def __exit__(self, exc_type, exc_val, exc_tb):
     self.close()
 
+  async def __aenter__(self):
+    return self
+
   async def __aexit__(self, exc_type, exc_val, exc_tb):
     await self.close()