Fix error code reordering to maintain backward compatibility

[Copilot]

Summary of Changes

• Restored original error code mappings (codes 2-6) in From<DoubleZeroError> for ProgramError and From<u32> for DoubleZeroError implementations
• The previous implementation reordered these mappings, which would cause deployed clients to misinterpret error codes returned by the smart contract
• Preserved the new ImmutableField error variant at code 68

Corrected mappings:

// Before (breaking)
2 => InvalidExchangePubkey
3 => InvalidDevicePubkey  
4 => InvalidLocationPubkey
5 => InvalidDeviceAPubkey
6 => InvalidDeviceZPubkey

// After (backward compatible)
2 => InvalidLocationPubkey
3 => InvalidExchangePubkey
4 => InvalidDeviceAPubkey
5 => InvalidDeviceZPubkey
6 => InvalidDevicePubkey

Testing Verification

• Verified error enum round-trip conversion with test_error_enum_conversions

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