Change ErrorCodeSerializer descriptor kind

[Leesin0222]

Summary of Changes

• Modified the ErrorCodeSerializer class by changing the PrimitiveSerialDescriptor type from PrimitiveKind.STRING to PrimitiveKind.INT.

Motivation and Context

• Problem Solved: When using PrimitiveKind.STRING in the ErrorCodeSerializer, errors could occur due to type mismatches during serialization and deserialization. Since ErrorCode.code is an Int, it should be handled consistently by using PrimitiveKind.INT.

Breaking Changes

• Change Impact: This change modifies the serialization and deserialization logic for ErrorCodeSerializer. If any existing code expects String type instead of Int for the ErrorCode values, those parts may need to be updated to handle the Int type properly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the [MCP Documentation](https://modelcontextprotocol.io)
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

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