Replace placeholder URL in L402 example with a working endpoint

The L402 example (`examples/l402.js`) uses a fake URL (`https://l402.example.com/protected-resource`) that isn't runnable as-is.

We should replace it with a real Alby L402 endpoint, similar to how the x402 example uses `https://x402.albylabs.com/demo/quote`.

Ref: PR #320 review comment from @rolznz

Open examples/l402.js and inspect the placeholder URL used by the L402 example. Compare it with the x402 example's working endpoint, then identify a real Alby L402 endpoint and verify that the example runs against it; done means the example no longer uses a fake URL.

Written by the indexing model from the issue text.