fix: idempotent voucher replay + 204-safe receipt wrapping

[Slokh]

Summary

Fixes two session/SSE production issues: idempotent voucher replay handling and 204-safe SSE receipt wrapping.

Problems & Solutions

1. Duplicate voucher submissions with the same cumulativeAmount were rejected with 402 verification-failed in legitimate retry/race paths.
   Solution: reject < highestVoucherAmount, accept == highestVoucherAmount as idempotent replay success after signature verification, and keep normal advancement rules for > highestVoucherAmount.
2. Successful management responses (204) could crash in SSE transport due to body-wrapping on null-body statuses.
   Solution: return null-body status responses (101/204/205/304) directly and only stream-wrap responses where a body is valid.

Validation

• pnpm build
• Added/updated targeted tests for voucher replay behavior and 204 receipt wrapping path

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/mppx@221

commit: 001c256