A2A Agents don't seem to support streaming via "agentcore invoke"

### Description

Not sure if this is supposed to be a bug or intended behavior.

This document (https://a2a-protocol.org/latest/topics/streaming-and-async/) suggests streaming may be supported by A2A servers (agentcore runtimes in our case). It seems that is not the case as of v0.25. Are there plans to support it (like with HTTP agents)?

### Steps to Reproduce

1. Run "agentcore add"
2. Create a HTTP and an A2A agent.
3. Run "agentcore deploy"
4. Run "agentcore invoke --runtime --stream" on each

### Expected Behavior

Both the HTTP agent and the A2A agent should stream the result.

### Actual Behavior

Only the HTTP agent does so. A2A sends the entire response at once.

### CLI Version

0.25.0

### Operating System

macOS

### Additional Context

_No response_

Start by reproducing the `agentcore add`, `agentcore deploy`, and `agentcore invoke --runtime --stream` flows for both HTTP and A2A agents on CLI version 0.25.0. Compare the streaming behavior and trace the CLI entry point handling `--stream` for each agent type; done means the A2A behavior is clarified and either streams like HTTP or is explicitly documented as unsupported.

Written by the indexing model from the issue text.