[codex] feat: add problem detail hints

[EfeDurmaz16]

Summary

• Add optional hint support to PaymentException problem details.
• Include default wallet/action hints for PaymentRequiredException, MalformedCredentialException, and PaymentMethodUnsupportedException.
• Cover hint inclusion and omission with focused unit tests.

Why

This brings mpp-java in line with the mpp-rb problem-details behavior added in stripe/mpp-rb#33. The structured hint gives clients and agents a recovery path when payment credentials are missing, malformed, or use an unsupported method, without changing existing problem details for errors that do not define a hint.

Verification

• mise exec java@temurin-17 -- ./gradlew test
• git diff --check

Notes

• This is an SDK parity/usability change, not a vulnerability report.