[workspace] Upgrade Soroban SDK to 25.3.0

Based on `SECURITY_AUDIT_VAULT_CONTRACT-V1.0.0.md` (finding F-17).

**Severity:** Enhancement

## Summary
Update `soroban-sdk` from 23.1.1 to 25.3.0 (or latest stable).

## Acceptance criteria

- [ ] Update `soroban-sdk` in workspace `Cargo.toml`
- [ ] Resolve any breaking changes
- [ ] All contracts build and tests pass
- [ ] Scout no longer reports `soroban_version` warning