feat(examples): adopt GameHarness for scenario-based testing in reference examples

[salazarsebas]

Background

cougr-core 1.1.0 provides test::GameHarness, Scenario, and ReplayLog behind the testutils feature. spawn_and_move includes sandbox_tests.rs demonstrating this pattern. Other reference examples rely on ad-hoc test helpers, which limits their utility as testing references for external developers.

Objective

Introduce GameHarness-based scenario tests in high-traffic reference examples to establish a consistent, replayable testing pattern.

In scope — Phase 1

Examples: spawn_and_move, tic_tac_toe, snake, battleship

Per example:

1. Add a test module (sandbox_tests.rs or test.rs) using GameHarness and Scenario.
2. Implement at least one multi-step scenario test (minimum three actions with intermediate assertions).
3. Document the testing approach in README.md.
4. Declare cougr-core = { version = "1.1.0", features = ["testutils"] } under dev-dependencies.

Out of scope

• Adoption across all 39 examples (future phases).
• Changes to production contract code beyond what tests require.

Definition of done

• cargo test passes with harness-based scenario tests in all four examples.
• Each README.md documents GameHarness as the recommended testing approach.
• Redundant custom test fixtures superseded by GameHarness are removed.

[felladaniel36-hash]

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[KevinMB0220]

@KevinMB0220 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I’m Kevin a full-stack and smart contract engineer building on Stellar. I specialize in designing and shipping scalable, secure systems end-to-end, from Soroban smart contracts to production-ready backend infrastructure and user-facing applications.

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[od-hunter]

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Hi, can I be assigned this issue please? I’ve gone through the issue carefully and I’m up for the task, kindly assign me please

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