test(contracts): add DataKey storage layout regression tests

[collinsezedike]

Summary

• Creates contracts/invisible_wallet/src/storage/tests.rs with one test per DataKey variant
• An exhaustive match on DataKey causes a compile error if any variant is added without updating the tests
• Unit variants are asserted against an exact ScVal::Vec([Symbol("Name")]) — any rename breaks the assertion
• Signer(BytesN<65>) is tested with a deterministic byte vector; Allowance(AllowanceKey) pins both the discriminant symbol and the inner map field order (spender → token)

Closes #259

Test plan

• cargo build -p invisible-wallet passes (CI gate)
• Renaming any DataKey variant causes a test assertion failure
• Adding a new variant without updating _cover_all_variants causes a compile error

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