[TypeScript] Support A2A protocol in AgentCore CLI + serve_a2a equivalent in TS SDK

[MeidanNasi]

Description

Summary

The AgentCore CLI (@aws/agentcore) and the TypeScript SDK (bedrock-agentcore)
do not currently support the A2A (Agent-to-Agent) protocol, while the Python
tooling does. This forces TypeScript users into a manual workaround to deploy
an A2A agent.

Environment

• CLI: @aws/agentcore (npm)
• SDK: bedrock-agentcore (npm, TypeScript)
• Language: TypeScript
• Protocol: A2A

Current behavior

1. Running agentcore and selecting TypeScript offers only the default
   HTTP protocol — there is no option to select A2A.
2. Selecting Python does offer the A2A protocol option.
3. The Python SDK exposes serve_a2a from bedrock_agentcore.runtime, but the
   npm package bedrock-agentcore/runtime has no serve_a2a equivalent.
4. Attempting --language TypeScript --protocol A2A is gated at the CLI level
   (per the CLI CHANGELOG: "gate MCP and A2A protocols behind Python-only until
   TS templates land").

Expected behavior

TypeScript should have parity with Python for A2A:

• agentcore CLI should allow selecting the A2A protocol for TypeScript
  agents and scaffold an appropriate template.
• The TypeScript SDK should provide a serve_a2a equivalent (or documented
  helper) that satisfies the A2A protocol contract (port 9000, POST /,
  GET /ping, GET /.well-known/agent-card.json).

Current workaround

A2A works with TypeScript today, but only manually:

• Implement the A2A contract using the official @a2a-js/sdk on port 9000.
• Register the agent as HTTP, then edit agentcore/agentcore.json to set
  protocol: A2A.
• Provide a custom ARM64 Dockerfile (base node:22-slim, EXPOSE 9000).
• Deploy via agentcore deploy (CodeBuild handles the ECR push), or bypass the
  CLI entirely with CDK (protocolConfiguration: ProtocolType.A2A).

This confirms the gap is a tooling/SDK limitation, not an architectural one —
AgentCore's A2A support is a transparent JSON-RPC proxy that is language-agnostic.

Requests

1. Add A2A protocol support (CLI scaffolding + template) for TypeScript.
2. Add a serve_a2a equivalent to the TypeScript SDK.
3. Document the TypeScript A2A path in the AgentCore devguide.

References

• Deploy A2A servers in AgentCore Runtime: https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-a2a.html
• A2A protocol contract: https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/runtime-a2a-protocol-contract.html
• A2A JS SDK: https://github.com/a2aproject/a2a-js

Acceptance Criteria

Acceptance Criteria

• Running the agentcore CLI with --language TypeScript offers A2A
  as a selectable protocol (not just HTTP).
• agentcore ... --language TypeScript --protocol A2A no longer errors out
  (the Python-only gate is removed for A2A).
• The CLI scaffolds a working TypeScript A2A template that satisfies the
  A2A protocol contract (port 9000, POST /, GET /ping,
  GET /.well-known/agent-card.json).
• The TypeScript SDK (bedrock-agentcore) exposes a serve_a2a equivalent
  (or a documented helper) so users don't have to wire up @a2a-js/sdk
  manually.
• A scaffolded TypeScript A2A agent can be deployed via agentcore deploy
  and successfully responds to a JSON-RPC message/send request.
• The AgentCore devguide documents the TypeScript A2A path end-to-end.

Additional Context

No response