fix(client): align send_message signature with BaseClient#740

Akshat8510 wants to merge 2 commits intoa2aproject:1.0-deva2aproject/a2a-python:1.0-devfrom

Akshat8510:fix/align-client-signatureAkshat8510/a2a-python:fix/align-client-signatureCopy head branch name to clipboard