feat: add mock score registry contract

[snkk2x-collab]

Summary

• add a test-only Soroban mock score registry contract with configurable score and tier responses
• add a server test helper for swapping SCORE_REGISTRY_CONTRACT_ID during local integration tests
• document how the mock contract fits into the local contract and interface layout

Validation

• PATH="$HOME/.cargo/bin:$PATH" cargo test
• pnpm --dir Server exec vitest run src/test-helpers/__tests__/mock-soroban.test.ts
• pnpm --dir Server exec vitest run
• pnpm --dir Server exec tsc --noEmit --declaration false