fix: add client-side challenge expiration validation

[JasonOA888]

Summary

Fixes #198

Problem

When a client receives a Challenge with an expires timestamp that has already passed, the client still attempts to create a credential and make the payment. This results in:

1. Wasted user time - wallet interactions that will be rejected
2. Unnecessary network round-trips
3. User confusion (why did my payment fail?)

The server already validates expiration (src/server/Mppx.ts:359-367), but by then the client has already connected wallet and signed transactions.

Solution

Adds client-side expiration validation at two code paths:

1. Fetch.from() - auto-402 interceptor, after challenge matching
2. Mppx.createCredential() - manual API, after challenge parsing

Both throw PaymentExpiredError (already defined in Errors.ts) when challenge.expires is set and in the past.

Changes

// src/client/internal/Fetch.ts - after finding matching challenge
validateChallengeExpiration(challenge)

// src/client/Mppx.ts - in createCredential()
if (challenge.expires && new Date(challenge.expires) < new Date()) {
  throw new Errors.PaymentExpiredError({ expires: challenge.expires })
}

Testing

// Expired challenge
const challenge = { expires: "2025-01-01T00:00:00Z", ... }
await mppx.createCredential(response)
// Throws: PaymentExpiredError: Payment expired at 2025-01-01T00:00:00Z.

Impact

• Type: UX improvement
• Risk: Low - fails faster with clearer error
• Breaking: No - just changes when the error is thrown