Implement gitlab_search MCP tool

Problem

Following the implementation of the gitlab_issue_search MCP tool in issue #566143 (closed), which enables issue searching across project, group, and instance scopes, we now face a scalability challenge as we expand search capabilities to other GitLab entities. The current approach of creating individual MCP tools for each scope (issues, merge requests, milestones, etc.) will result in exponential tool proliferation.

Proposal

Let's implement a unified search tool for MCP server that consolidates access to multiple search APIs and provides a streamlined interface for search functionality:

There are 3 search APIs available:

• Search global
• Search group
• Search project

Let's expose code search as the first iteration.

Desired Outcome

• Implementation: Working consolidated search tool with code search functionality
• Documentation: Technical decision log covering:
  • Architecture choice rationale (aggregated vs. custom tool)
  • Permission handling strategy and implementation
  • etc.