Fix Version Marker Pattern test examples to use proper contract context

[Copilot]

The test examples in the Version Marker Pattern section accessed env.storage() directly from Env::default() without a contract context, which panics at runtime. All storage reads/writes and helper calls must occur inside env.as_contract(...).

Changes

• Each test now registers a contract via env.register(Contract, ()) to obtain a contract_id
• All storage and helper calls wrapped in env.as_contract(&contract_id, || { ... }) — including legacy v1 state injection, read_data, write_data, and version key reads
• Prose updated to explain why env.register + env.as_contract are required

#[test]
fn test_reads_v1_entry_as_v2() {
    let env = Env::default();
    let contract_id = env.register(Contract, ());
    let id: u32 = 42;

    env.as_contract(&contract_id, || {
        let v1_data = DataV1 { a: 10, b: 20 };
        env.storage().persistent().set(&DataKey::Data(id), &v1_data);
    });

    let result = env.as_contract(&contract_id, || read_data(&env, id));

    assert_eq!(result.a, 10);
    assert_eq!(result.b, 20);
    assert_eq!(result.c, None);
}

Pattern now matches the Versioned Enum Pattern tests already present in the same guide.

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