refactor: adapt wrong imports in tck and sample (#948)

# Description

This PR fixes the wrong imports after the introduction of new
default_request_handler_V2

Author: guglielmo-san

samples/hello_world_agent.py

@@ -12,10 +12,7 @@
 from a2a.server.agent_execution.agent_executor import AgentExecutor
 from a2a.server.agent_execution.context import RequestContext
 from a2a.server.events.event_queue import EventQueue
-from a2a.server.request_handlers import GrpcHandler
-from a2a.server.request_handlers.default_request_handler import (
-    DefaultRequestHandler,
-)
+from a2a.server.request_handlers import DefaultRequestHandler, GrpcHandler
 from a2a.server.routes import (
     create_agent_card_routes,
     create_jsonrpc_routes,

tck/sut_agent.py

@@ -17,9 +17,7 @@
 from a2a.server.agent_execution.agent_executor import AgentExecutor
 from a2a.server.agent_execution.context import RequestContext
 from a2a.server.events.event_queue import EventQueue
-from a2a.server.request_handlers.default_request_handler import (
-    DefaultRequestHandler,
-)
+from a2a.server.request_handlers import DefaultRequestHandler
 from a2a.server.request_handlers.grpc_handler import GrpcHandler
 from a2a.server.routes import (
     create_agent_card_routes,