refactor: implement unified CoreError enum and migrate 14 contracts

[benedictworks-home]

Title: Refactor: Implement Unified CoreError Enum and Migrate 14 Contracts

Description: This PR refactors the Soroban contract error handling system from raw u32 values to a standardized CoreError enum, ensuring consistent cross-contract error handling.

Changes Made:

1. Created contracts/types/src/errors.rs :

   • Defined the CoreError enum with variants for Auth, Initialization, Validation, Payment, State, Storage, External, and Recovery errors
   • Marked old u32 error codes as deprecated for backward compatibility
   • Added methods for user-friendly messages, error codes, and event emission

2. Updated contracts/types/src/lib.rs :

   • Exported the errors module and CoreError enum

3. Migrated All 14 Contracts:

   • Added bidirectional From conversions between each contract's specific error enum and CoreError
   • Contracts updated: subscription, credit, oracle, batch, metering

4. Updated Documentation:

   • Enhanced developer-portal/docs/api-reference.md with CoreError enum details

Key Benefits:

• Consistent error handling across all contracts
• Clear error categories for debugging
• Standardized event emission for errors
• Backward compatibility maintained
• Auto-generatable documentation schema

Closes #578

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