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

Scope

This PR is now a single commit on top of main and only touches these related test files:

• src/tempo/server/Session.test.ts
• test/tempo/session.ts
• src/middlewares/express.test.ts
• src/middlewares/hono.test.ts
• src/middlewares/elysia.test.ts
• src/middlewares/nextjs.test.ts

Notes

• adapter-level fee-payer coverage is intentionally kept to smoke tests
• deeper fee-payer validation remains in the core suites, especially src/tempo/server/Session.test.ts, src/tempo/server/Charge.test.ts, and src/client/internal/Fetch.test.ts
• no unrelated harness or setup changes remain in the PR

Testing

• pnpm check:ci
• pnpm check:types
• pnpm test

[pkg-pr-new]

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

commit: 0433ac6