refactor: alien-gateway extract storage key builders into storage.rs

[ryzen-xp]

📝 Description

Storage keys like vec![Symbol::new(env, StellarAddress), address] are constructed inline. Any typo creates a silent wrong key. Create alien_gateway/storage.rs with stellar_address_key(addr), smt_root_key(), owner_key(), username_key(), created_at_key() helpers.

✅ Requirements

• Storage keys like vec![Symbol::new(env, StellarAddress), address] are constructed inline
• Any typo creates a silent wrong key
• Create alien_gateway/storage.rs with stellar_address_key(addr), smt_root_key(), owner_key(), username_key(), created_at_key() helpers.

🎯 Acceptance Criteria

• Implementation matches the description above
• Tests added or updated to cover the new behaviour
• cargo test and cargo clippy pass with no warnings
• CI (Smart Contracts CI) passes

📁 Expected files to change

• gateway-contract/contracts/core_contract/src/lib.rs
• gateway-contract/contracts/core_contract/src/test.rs

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💬 Commit: refactor: alien-gateway extract storage key builders into storage.rs
🌿 Branch: refactor/alien-gateway-extract-storage-key-builders-into-st | Priority: LOW | Difficulty: ☕ one-coffee

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Type: Refactor · Priority: LOW · Difficulty: ☕ one-coffee · Track: Contract

[Stephan-Thomas]

@Stephan-Thomas has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

hi, i would like to work on this.

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Congratulations, @Stephan-Thomas! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

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This issue has been completed by @Stephan-Thomas as part of the Stellar Wave Program's 3rd Wave 🥳

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