feat(x402-buyer): expose confirm-spend persistence failures

[bussyjd]

Summary

Adds an explicit buyer-side metric for successful requests whose consumed-auth state could not be persisted.

Why

Production-readiness testing found a live mismatch between:

• payment_attempts_total
• payment_success_total
• auth_spent

A likely explanation is that ConfirmSpend() can fail after a successful upstream response, causing the live gauges to drift after reload. This PR makes that path directly observable.

What changed

• added obol_x402_buyer_confirm_spend_failure_total
• increment the metric when ConfirmSpend() fails after an upstream success
• keep the request successful; this PR is observability-first, not a behavioral change
• added a regression test using an unwritable state path to force the persistence failure path

Validation

• go test ./internal/x402/buyer