Upgrade soroban-sdk and refresh the test snapshots #308

Where: contracts/Cargo.toml, contracts/showcase/test_snapshots/
Problem: The workspace pins soroban-sdk = "25", and Dependabot PR #188 (to 29) can't merge as-is because the snapshot JSON changes and the API may have moved.
Change: Upgrade, fix any API changes, regenerate snapshots, and confirm Prism still parses the new spec output (events in particular).
Done when: CI's contracts job passes on the new SDK and the frontend spec tests still pass on the rebuilt WASM.

Found by reading the codebase at commit 00798b6 on main. File and line references are to that commit.