feat(a2a): support custom ClientFactory in A2AAgent for authenticated requests

[cogwirrel]

Motivation

The A2AAgent currently always creates a default ClientFactory internally, providing no way to customize how HTTP requests are made to the remote A2A server. This prevents users from authenticating requests with mechanisms like SigV4 signing or bearer tokens, which is required when invoking A2A agents on services like Bedrock AgentCore.

The Python SDK already supports this by accepting a custom A2A client factory in its A2AAgent class.

Resolves: #802

Public API Changes

A2AAgentConfig now accepts an optional clientFactory parameter:

// Before: no way to authenticate requests
const agent = new A2AAgent({ url: "http://localhost:9000" })

// After: pass a custom ClientFactory for authenticated requests
const sigv4Fetch = ...
const a2aClientFactory = new ClientFactory(
  ClientFactoryOptions.createFrom(ClientFactoryOptions.default, {
    transports: [new JsonRpcTransportFactory({ fetchImpl: sigv4Fetch })],
    cardResolver: new DefaultAgentCardResolver({ fetchImpl: sigv4Fetch }),
  }),
)

const agent = new A2AAgent({
  url: "http://localhost:9000",
  clientFactory: a2aClientFactory,
})

When no clientFactory is provided, the default ClientFactory is used — fully backward compatible.

Use Cases

• SigV4 authentication: Sign requests when invoking A2A agents on Bedrock AgentCore
• Bearer token authentication: Pass JWT or other bearer tokens for authenticated endpoints
• Custom transport: Provide any custom fetch implementation for specialized networking requirements

[poshinchen]

This looks neat to me