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

### 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_

Start by tracing the CLI's TypeScript protocol selection and A2A gate, then inspect the TypeScript SDK runtime and the existing Python A2A behavior. Use the A2A protocol contract and @a2a-js/sdk references to define the scaffold and helper. Done means the CLI scaffolds and deploys a TypeScript agent that serves the required endpoints, handles message/send, and is documented in the devguide.

Written by the indexing model from the issue text.