Fix JavaScript SDK import path and authentication

[alenkacz]

The example was using incorrect import path:

• Old: import { A2AClient } from '@a2a-js/sdk'
• New: import { ClientFactory, ClientFactoryOptions } from '@a2a-js/sdk/client'

The A2AClient class is not exported from the main '@a2a-js/sdk' package, it's available in the '/client' subpath export.

Also updated to use proper SDK authentication pattern with CallInterceptor as recommended in the official SDK documentation.

Fixes the "A2AClient is not exported" error that users would encounter when trying to run the example.

[malinskibeniamin]

there is another thing we should do, you can patch the fetch API for the agent card to ensure the JWT token is provided properly, I did this for our implementation in Console UI, but maybe the API changed since few months ago.

    const fetchWithCustomHeader: typeof fetch = async (url, init) => {
      const headers = new Headers(init?.headers);
      if (this.config.jwt) {
        headers.set('Authorization', `Bearer ${this.config.jwt}`);
      }
      headers.set('X-Redpanda-Stream-Tokens', 'true');

      const newInit = { ...init, headers };

      return fetch(url, newInit);
    };

    const client = await createA2AClientWithFallback(this.modelId, { fetchImpl: fetchWithCustomHeader });

then

async function createA2AClientWithFallback(agentUrl: string, options: A2AClientOptions): Promise<A2AClient> {
  const urls = getAgentCardUrls({ agentUrl });
  const errors: Error[] = [];

  for (let i = 0; i < urls.length; i++) {
    const url = urls[i];
    const isLastUrl = i === urls.length - 1;

    try {
      const client = await A2AClient.fromCardUrl(url, options);
      return client;
    } catch (error) {
      errors.push(error as Error);
      console.log(`Failed to fetch agent card from ${url}, ${isLastUrl ? 'no more URLs to try' : 'trying next URL...'}`);
      // Continue to next URL if not the last one
      if (isLastUrl) {
        break;
      }
    }
  }

then

export function getAgentCardUrls({ agentUrl }: { agentUrl: string }): string[] {
  // Normalize URL by removing trailing slash
  const normalizedUrl = agentUrl.endsWith('/') ? agentUrl.slice(0, -1) : agentUrl;

  // If the URL already points to a specific file, use it as-is
  const isFullPath =
    normalizedUrl.includes('agent-card.json') ||
    normalizedUrl.includes('agent.json') ||
    normalizedUrl.includes('well-known');

  if (isFullPath) {
    return [normalizedUrl];
  }

  // Try agent-card.json first, fall back to agent.json
  return [`${normalizedUrl}/.well-known/agent-card.json`, `${normalizedUrl}/.well-known/agent.json`];
}