fix: return updated agent_card in JsonRpcTransport.get_card() (#552)

## Fixes #551 

### Changes
- Fixed `JsonRpcTransport.get_card()` to return the newly fetched
authenticated extended card instead of the stale card

### Details
Changed line from `return card` to `return self.agent_card` to ensure
the method returns the updated card after fetching the authenticated
extended version.

This aligns the JsonRpcTransport behavior with RestTransport's correct
implementation.

### Testing
- Verified the fix matches RestTransport's pattern
- Confirmed internal state and return value are now consistent

Release-As: 0.3.18

Author: ShishirRmc

src/a2a/client/transports/jsonrpc.py

@@ -414,7 +414,7 @@ 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."""

tests/client/transports/test_jsonrpc_client.py

@@ -774,7 +774,7 @@ async def test_get_card_with_extended_card_support(
             mock_send_request.return_value = rpc_response
             card = await client.get_card()
 
-        assert card == agent_card
+        assert card == AGENT_CARD_EXTENDED
         mock_send_request.assert_called_once()
         sent_payload = mock_send_request.call_args.args[0]
         assert sent_payload['method'] == 'agent/getAuthenticatedExtendedCard'