fix(mcp): add content to structuredContent

Description

Add content inside the structuredContent MCP result, so the Claude correctly identifies tool output.

Fix the error in VS Code Copilot Chat: Failed to validate tool mcp_local_gitlab__glab_api: Error: tool parameters array type must have items. Please open a Github issue for the MCP server or extension which provides this tool.

Related Issues

Resolves #8104

How has this been tested?

Tested in VS Code with GitHub Copilot Chat 0.36.2.

VS Code version

Version: 1.108.2
Commit: c9d77990917f3102ada88be140d28b038d1dd7c7
Date: 2026-01-21T13:52:09.270Z
Electron: 39.2.7
ElectronBuildId: 12953945
Chromium: 142.0.7444.235
Node.js: 22.21.1
V8: 14.2.231.21-electron.0
OS: Darwin arm64 25.2.0

1. make build
2. Add .vscode/mcp.json:

{
    "servers": {
        "Local GitLab MCP": {
            "type": "stdio",
            "command": "/Users/alexandear/src/gitlab.com/gitlab-org/cli/bin/glab",
            "args": [
                "mcp",
                "serve"
            ]
        },
    },
    "inputs": []
}

3. Start Local GitLab MCP.
4. Open Copilot Chat and prompt: "Give me the list of the last 5 PRs? Use glab mcp".

Before this PR:

Failed to validate tool mcp_local_gitlab__glab_api: Error: tool parameters array type must have items. Please open a Github issue for the MCP server or extension which provides this tool

After:

Screenshots (if appropriate):