Implement create_merge_request MCP Tool and service

Proposal

Add support for new MCP tool: create_merge_request which maps to Merge requests API - Create MR

• Add a new API mapping following our framework/annotation approach
• Allow the following arguments from the API documentation
  • id (required) - The ID or URL-encoded path of the project
  • source_branch (required) - The source branch
  • target_branch (required) - The target branch
  • title (required) - Title of MR
  • allow_collaboration (optional) - Allow commits from members who can merge to the target branch
  • assignee_id (optional) - Assignee user ID
  • assignee_ids (optional) - The ID of the users to assign the merge request to
  • description (optional) - Description of the merge request. Limited to 1,048,576 characters
  • labels (optional) - Labels for the merge request, as a comma-separated list
  • milestone_id (optional) - The ID of a milestone to assign the merge request to
  • remove_source_branch (optional) - Flag indicating if a merge request should remove the source branch when merging
  • reviewer_ids (optional) - The ID of the users to request a review from
  • squash (optional) - Flag indicating if the merge request should squash commits when merging
  • target_project_id (optional) - The ID of the target project

route_setting :mcp, tool_name: :create_merge_request, params: [:id]