SC-22: Standardize error codes across all Soroban contracts

[chinweobtagaz]

Context: Currently, contracts return generic panics or inconsistent custom error types, making frontend debugging and error handling difficult.
Location: contracts/game_contract/src/error.rs
Acceptance Criteria:

• Define a centralized #[contracterror] enum with distinct u32 values for Unauthorized, InvalidAmount, BadState, etc.
• Replace all panic! calls with proper error returns.

[ZeePearl56]

@ZeePearl56 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Kindly assign

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[Venrable18]

@Venrable18 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Good day maintainer..

I’m a full-stack and test integration developer…. 

Please assign this task to me…



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[NEA-DEV-coder]

@NEA-DEV-coder has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi maintainer, I would like to work on this issue

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[chidii]

@chidii has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello Maintainer, I've reviewed this issue thoroughly and I'm well positioned to resolve it — experienced across the relevant stack and comfortable navigating the existing codebase. Ready to start right away.

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[drips-wave]

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