Add Streaming Support to A2A

[physicsrob]

This PR implements streaming capabilities for pydantic-ai's A2A integration, enabling real-time updates during agent execution.

• Streaming Agent Execution: Switched from blocking agent.run() to streaming agent.iter() to emit real-time updates
• Real-time Status Updates: Agents now emit TaskStatusUpdateEvent messages for working/completed states
• Incremental Message Streaming: Agent responses are streamed as they're generated when enable_streaming=True
• Artifact Updates: Final results are emitted as TaskArtifactUpdateEvent for structured outputs

[ ] Blocked by pydantic/fasta2a#26 -- This PR relies on functionality introduced in my previous fasta2a PR

[physicsrob]

cc @Kludex

FYI, the next PR is ready to go here (physicsrob#1). This will add dependency injection support. But first we need to land the fasta2a change and this current PR.

[aristide1997]

@Kludex have you had a chance to look at this?