fix: return updated extended card in JsonRpcTransport.get_card()

The get_card() method was incorrectly returning a stale local variable
'card' instead of the updated 'self.agent_card' after fetching the
authenticated extended card from the server.

This caused callers to receive outdated basic card information instead
of the extended card with additional authentication capabilities.

- Line 316: Changed 'return card' to 'return self.agent_card'
- Fixes critical logic bug where extended card data was fetched but not returned

Author: chaturvedakash

src/a2a/client/transports/jsonrpc.py

@@ -424,7 +424,7 @@ async def get_card(
 
         self.agent_card = card
         self._needs_extended_card = False
-        return card
+        return self.agent_card
 
     async def close(self) -> None:
         """Closes the httpx client."""