fix: add BaseClient to imports and __all__ in client module

Author: martimfasantos

src/a2a/client/__init__.py

@@ -7,6 +7,7 @@
     CredentialService,
     InMemoryContextCredentialStore,
 )
+from a2a.client.base_client import BaseClient
 from a2a.client.card_resolver import A2ACardResolver
 from a2a.client.client import Client, ClientConfig, ClientEvent, Consumer
 from a2a.client.client_factory import ClientFactory, minimal_agent_card
@@ -51,6 +52,7 @@ def __init__(self, *args, **kwargs):
     'A2AClientTimeoutError',
     'A2AGrpcClient',
     'AuthInterceptor',
+    'BaseClient',
     'Client',
     'ClientCallContext',
     'ClientCallInterceptor',