test: fix nonce_retry mock to include use_dpop_nonce body (C2 followup)
The nonce_retry_sends_exactly_two_requests test was failing because the mock
returned a bare 400+DPoP-Nonce with no JSON body. After the C2 fix, execute_with_retry
checks the response body for {"error":"use_dpop_nonce"} before retrying; empty body
caused the code to return Err(NotAuthenticated) instead of triggering the retry.