Harden run-code-block server readiness and screenshot navigation retries

[Copilot]

run:server:start could report success before the local app was actually accepting connections, causing screenshot visit steps to fail intermittently. Retry handling also wasn’t resilient enough for transient startup/network races.

• Server startup gating

  • Passes directive timeout through to server.start(...) (was parsed but not applied).
  • After log-based startup detection (expect), extracts/validates URL and blocks until the host/port accepts TCP connections.
  • Fails with stdout/stderr context and tears down the spawned server on readiness timeout.

• Navigation retry hardening

  • Expands retryable page.goto(...) failures to include additional transient conditions (ERR_ABORTED, HTTP response failure case).
  • Increases retry budget and adds bounded exponential backoff between attempts.
  • Recreates page context between attempts to avoid stale/invalid browser state.

• Representative change

  let stdout = await server.start(args.expect, args.timeout);
  
  let readyURL = extractURL(args.expect);
  if (readyURL) {
    await waitForServerReady(readyURL, args.timeout || 30000);
  }