test: widen fee-payer server coverage

[brendanjryan]

Summary

• widen fee-payer coverage in the Tempo session server suite across direct and fee-payer modes
• teach the shared session test transaction helpers to build fee-payer open and top-up transactions
• reduce middleware adapter coverage to one fee-payer smoke test per surface while keeping exhaustive fee-payer behavior in the core session and charge suites

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/mppx@252

commit: 9876ce3