fix(server): return a fresh challenge on intent.verify failure instead of raising

[ygd58]

What

verify_or_challenge caught exceptions from intent.verify(credential, request) only to emit payment_failed and then Kernel.raise — the only failure path in this function that re-raises instead of returning a fresh challenge via new_challenge.call. Every other failure case (challenge id mismatch, binding mismatch, expired challenge, request mismatch, etc.) already returns new_challenge.call(credential, error, echo), producing a retryable 402. A method verification failure — a typed PaymentError (declined, insufficient funds) or an unexpected error — escaped instead.

Flagged by the cross-SDK audit as AGR-2026-089. Canonical mppx's createMethodFn wraps method verification the same way every other failure path here already does. This is the same gap I already fixed in stripe/mpp-java's Verify.java (#26).

Fix

Replace the rescue block's manual emit_payment_failed + Kernel.raise with return new_challenge.call(credential, e, echo). new_challenge.call already emits payment_failed internally when passed a non-nil error (see its own definition earlier in the file), so the manual emit_payment_failed call is now redundant — removed rather than left as a duplicate emission.

Testing

Adds three regression tests in test/mpp/test_server.rb, using a new ThrowingIntent test double (mirrors the existing MockIntent):

• A typed VerificationFailedError from intent.verify returns a fresh Mpp::Challenge instead of raising.
• A plain RuntimeError (not a typed PaymentError) also returns a fresh challenge rather than escaping uncaught — the "unexpected error" half of the fix.
• payment_failed fires exactly once (not twice) for a verify failure, confirming the old manual emit call was removed rather than duplicated alongside new_challenge.call's own emission.

I couldn't run the test suite in my sandbox — no access to rubygems.org to install bundler/minitest — but ruby -c confirms both files parse cleanly. Please run bundle exec rake test before merging, happy to fix anything that doesn't match.

Fixes tempoxyz/mpp-tools#211

[raubrey-stripe]

Build failure @ygd58 if you don't mind taking one more pass 🙏

[ygd58]

Thanks for the ping — the branch was rebased onto an older verify.rb, before #73/#74 migrated the call site from intent.verify(...) directly to IntentLifecycle.call(intent, credential, request). That's what CI was failing on. Rebased onto current main and reapplied the same fix against the new call site (the underlying bug — and this fix — is unchanged, IntentLifecycle falls back to the legacy #verify path for intents like the test double this PR's regression tests use). Should be green now.