Add optional fields to create_merge_request MCP tool

Summary

This MR adds optional fields to the create_merge_request MCP tool to match the functionality available in the create_issue tool.

Changes

• Added assignee_ids, reviewer_ids, description, labels, and milestone_id fields to the create_merge_request MCP tool
• Created create_merge_request_mcp_params helper method in MergeRequestsHelpers to define the MCP parameters
• Updated the route_setting in the create_merge_request endpoint to use the new helper method
• Added comprehensive tests for the new fields in both CE and EE test specs

Testing

• Connect to https://docs.gitlab.com/development/mcp_server/
• Execute create_merge_request tool with the new fields