Python: [A2A] Respect server-side `blocking=false` instead of local polling

[SergeyMenshykh]

Context

.NET PR #5423 sets SendMessageConfiguration.ReturnImmediately = true when the caller opts into background responses (A2AAgent.cs#L117).

Current Python state

python/packages/a2a/agent_framework_a2a/_agent.py ~L296 sends without any MessageSendConfiguration. The background=True flag is honored only client-side inside _map_a2a_stream, so the server is never told to return immediately.

Proposed change

• When background=True, set SendMessageConfiguration(blocking=False) on the outgoing SendMessageRequest.
• Stop using streaming (send_message SSE) for non-streaming operations.