feat: introduce standardized contract error codes and replace string …

[oyinade247]

closes #348

Pull Request: Centralize Contract Panics into Definitive Error Enum

Summary This PR implements a centralized error handling system for the Soroban contracts. By replacing generic panic!() calls with a #[contracterror] enum, we enable frontend SDKs to receive specific u32 error codes. This allows for precise error reporting and user-friendly tooltips (e.g., "Bounty Not Found" instead of a generic "Panic").

Core Changes

• New Error Module: Created contract/src/errors.rs with the ErrorCode enum.
• Refactored Panics: Updated the following modules to use panic_with_error!:
  • lib.rs: Initialization checks.
  • bounty/mod.rs: Authorization, claim checks, and bounty lookups.
  • treasury/management.rs: Balance checks, budget enforcement, and treasury lookups.
• Documentation: Created contract/ERRORS.md to map u32 codes to descriptions and recommended UI tooltips for frontend developers.
• Reference Module: Added contract/src/dummy.rs to demonstrate the standard implementation pattern.

Mapped Error Codes

Code | Enum Value | Description -- | -- | -- 1 | Unauthorized | Permission denied for the caller. 2 | InsufficientFunds | Balance is too low for the transaction. 3 | BountyNotFound | Specified bounty ID does not exist. 4 | AlreadyInitialized | Contract setup has already been completed. 5 | TreasuryNotFound | Specified treasury ID does not exist. 6 | TransactionNotFound | Specified transaction ID does not exist. 7 | BudgetExceeded | Category-specific budget limit reached. 8 | AllowanceExceeded | Admin-specific allowance limit reached.

Testing Refactored logic maintains existing functional behavior while upgrading the revert mechanism to use native Soroban error codes. Verified that panic_with_error! is correctly implemented across the target modules.

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