Make credential_service parameter optional

Updated the VeA2AServer constructor and init_app function to allow credential_service to be optional by defaulting it to None. This increases flexibility for cases where credentials are not required.

Author: loveyana

veadk/a2a/ve_a2a_server.py

@@ -35,7 +35,7 @@ def __init__(
         url: str,
         app_name: str,
         short_term_memory: ShortTermMemory,
-        credential_service: BaseCredentialService,
+        credential_service: BaseCredentialService | None = None,
     ):
         self.agent_card = get_agent_card(agent, url)
 
@@ -69,7 +69,7 @@ def init_app(
     app_name: str,
     agent: Agent,
     short_term_memory: ShortTermMemory,
-    credential_service: BaseCredentialService,
+    credential_service: BaseCredentialService | None = None,
 ) -> FastAPI:
     """Init the fastapi application in terms of VeADK agent.