What happened?

AttributeError occurs when making a message/send call via JSON-RPC in A2A SDK v0.3.5.

Steps to Reproduce

1. Implementedan AgentExecutor according to the A2A standard

class MyAgentExecutor(AgentExecutor):
    async def execute(self, context: RequestContext, event_queue: EventQueue) -> None:
        # Agent logic here
        pass

    async def cancel(self, context: RequestContext, event_queue: EventQueue) -> None:
        # Cancel logic here  
        pass

2. Configured the server with A2AFastAPIApplication like below

executor = MyAgentExecutor()
task_store = InMemoryTaskStore()
queue_manager = InMemoryQueueManager()

request_handler = DefaultRequestHandler(
    agent_executor=executor,
    task_store=task_store,
    queue_manager=queue_manager
)

jsonrpc_handler = JSONRPCHandler(
    agent_card=agent_card,
    request_handler=request_handler
)

app = A2AFastAPIApplication(
    agent_card=agent_card,
    http_handler=jsonrpc_handler
)

3. and then
   AttributeError: 'MessageSendParams' object has no attribute 'params'

The JSON-RPC call should succeed and return an appropriate response but it didn't.
There were no problem with A2AFastAPIApplication tho.

Relevant log output

File "/.../a2a/server/request_handlers/jsonrpc_handler.py", line 106, in on_message_send
    task_or_message = await self.request_handler.on_message_send(
File "/.../a2a/server/request_handlers/jsonrpc_handler.py", line 107, in on_message_send
    request.params, context
    ^^^^^^^^^^^^^^
AttributeError: 'MessageSendParams' object has no attribute 'params'

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