fix: make soroban contract wasm-safe and stabilize giveaway tests

[Cedarich]

• Fix Soroban contract WASM build and giveaway tests

• Body:

• Ensure geev-core builds for wasm32-unknown-unknown by:

  • Enabling #![no_std] in the contract crate.
  • Disabling soroban-sdk default features for main dependencies.
  • Keeping soroban-sdk testutils only in dev-dependencies .

• Switch giveaway contract to use persistent storage instead of instance storage.

• Add a minimal test contract and register it in tests so storage access runs in a proper contract context.

• Use env.mock_all_auths() in tests so require_auth passes in the test host.

• Fix enter_giveaway tests to:

  • Set up giveaway state via env.storage().persistent() inside env.as_contract .
  • Verify success before end time, and panics for post-end and duplicate entries.
    Testing

• cargo test --workspace

• cargo build --workspace --target wasm32-unknown-unknown --release

close #149

[Kaylahray]

Arigato 🙋‍♀️