fix(samples): retry shell agent-card discovery during startup

[stablegenius49]

Summary

• add a shared retry/backoff helper for A2A Agent Card discovery in the lit shell sample
• reuse the same extension-aware fetch wrapper in both the browser client and dev middleware
• show a clearer first-load waiting message while the sample waits for agents to finish booting
• add focused retry unit tests for the new helper

Closes #587

How did you test this change?

• npm run build in samples/client/lit/shell
• node --test dist/tests/client-factory.test.js in samples/client/lit/shell

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[gemini-code-assist]

Code Review

This pull request introduces a robust retry mechanism with exponential backoff for agent card discovery, which is a great improvement for handling startup race conditions. The new logic is well-encapsulated in a2a-client-factory.ts, thoroughly tested, and cleanly integrated into both the Lit shell client and the dev middleware. The improved loading message is also a nice touch for the user experience. I have one suggestion to enhance the error reporting for better debuggability.

[gemini-code-assist]

When the retry loop exhausts all attempts, this logic correctly throws an error. However, if lastError is not an instance of Error (e.g., it's a plain object or a string from a library), the original error details are lost in the new generic Error message. This can make debugging more difficult.

To provide more context, you could serialize the lastError into the new error message. A simple if/else block would also improve readability over the ternary.

  if (lastError instanceof Error) {
    throw lastError;
  }
  throw new Error(
    `Failed to connect to agent card: ${cardUrl}. Last error: ${JSON.stringify(
      lastError,
    )}`,
  );