feat(tempo): add stateless session actions

[deodad]

Summary

Add stateless Tempo session actions stacked on the stateless charge PR.

Motivation

Accounts need the same signer-selection split for sessions that charge fill provides: construct Tempo calls first, pick the right signer or access key, then create the credential without reimplementing session channel details outside mppx.

Changes

• Added src/tempo/Session.ts with open, topUp, voucher, and close stateless actions.
• Exported the new actions under tempo.Session while preserving the existing session submodule namespace.
• Refactored src/tempo/client/Session.ts to use the stateless session actions internally.
• Added focused coverage in src/tempo/Session.test.ts for open/topUp fill plus credential creation for open, topUp, voucher, and close.

Testing

pnpm test src/tempo/Session.test.ts src/tempo/client/Session.test.ts

Result: 2 test files passed, 31 tests passed.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/mppx@486

commit: 60fef59