bugfix(gitlab): Make owner and fork properties optional to fix GitLab error "Invalid arguments"

[cong-toquang]

Description

Server Details

• Server: GitLab
• Changes to: N/A

Motivation and Context

This PR fixes bug #357.

How Has This Been Tested?

I created a local MCP server and verified that the local GitLab MCP server is able to retrieve information from a self-hosted GitLab instance via Cline in VSCode.

Breaking Changes

None.

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 Protocol Documentation
• My changes follow MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context