Fix: Return updated agent_card in JsonRpcTransport.get_card()

The get_card() method was returning the stale 'card' variable instead of the newly fetched 'self.agent_card' when retrieving authenticated extended cards. This caused callers to receive outdated agent metadata.
This fix aligns JsonRpcTransport with RestTransport's correct behavior.

Author: ShishirRmc

src/a2a/client/transports/jsonrpc.py

@@ -374,7 +374,7 @@ async def get_card(
             raise A2AClientJSONRPCError(response.root)
         self.agent_card = response.root.result
         self._needs_extended_card = False
-        return card
+        return self.agent_card
 
     async def close(self) -> None:
         """Closes the httpx client."""