test: add shared multi-user contract harness

[sa-salami]

Summary

• add a shared test_utils crate for token setup, funded users, deterministic ledger time, minting, and solvency assertions
• migrate the five contract test suites to the shared token helpers
• add a multi-user solvency example and the missing extern crate std declaration in the plans tests

Verification

• cargo test --workspace — 58 tests passed

Fixes #36

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[sa-salami]

@Lokt-In @sa-salami Fixed in commit e9facbd225790009b675c8e90b19deb428100482. Added the shared five-contract test harness, migrated token helpers, added deterministic ledger/solvency helpers, and included a multi-user solvency example. Verification: cargo test --workspace passed all 58 tests. The PR body references Fixes #36; please review and merge when CI completes.

[sa-salami]

Hi @Mackenzie-OO7 👋 Friendly reminder that this PR for #36 is ready for review — there are no merge conflicts and no outstanding change requests. Could you take a look and merge it when you get a chance? Happy to make any changes you'd like. Thank you!