[Feature Request] Allow for multiple A2A agents to run simultaneously with agentcore dev

[nxf5025]

Description

Currently when you have multiple A2A agents set in agentcore.json when you run agentcore dev it fails with Bind for :::9000 failed: port is already allocated.

This request is to remove the hardcoded port 9000 and potentially allow the port to be specified directly in the agentcore.json

Acceptance Criteria

• When multiple A2A agents are set in agentcore.json there should be no port conflicts
• Optionally allow for a use specified port to be set in agentcore.json

Additional Context

No response