Feature: Add update_merge_request tool

[sgaunet]

Add a new tool to update existing merge requests in a GitLab project.

Description

Implement an update_merge_request tool that allows updating merge request details.

Proposed Parameters

• project_path: GitLab project path (required)
• merge_request_iid: MR internal ID (required)
• title: Updated title (optional)
• description: Updated description (optional)
• state_event: State change (close, reopen) (optional)
• assignee_ids: Updated assignee IDs (optional)
• reviewer_ids: Updated reviewer IDs (optional)
• labels: Updated labels (optional)
• milestone_id: Updated milestone ID (optional)
• remove_source_branch: Update auto-remove setting (optional)
• draft: Convert to/from draft (optional)

Implementation Notes

• Use GitLab API endpoint: PUT /projects/:id/merge_requests/:merge_request_iid
• Return structured JSON with updated MR details
• Support partial updates (only update provided fields)
• Include proper validation and error handling