[DX] Build interactive `npx documentdb-mcp init` wizard

Goal

Replace the JSON-editing first-run with a 3-question wizard that writes the correct config to the right file.

What to ship

A new command npx @microsoft/documentdb-mcp init (or equivalent) that:

1. Detects the client (or asks: VS Code / Copilot CLI / Claude Desktop / other).
2. Asks for transport (stdio recommended for local).
3. Asks for the DocumentDB connection target (URI or Entra endpoint).
4. Writes the config to the correct location for the detected client (e.g. %APPDATA%/Code/User/mcp.json, ~/.config/copilot/...).
5. Prints next steps: Restart VS Code, then ask: 'list the databases I can access.'

Acceptance criteria

• Works on Windows, macOS, Linux.
• Detects existing config and offers to merge rather than overwrite.
• Validates the resulting config by running the server in --validate mode.
• Has at least one unit test per client target.

Why

First-run JSON escape is the single biggest friction point. init collapses it to a single command.

Depends on

• Tier 1: --validate flag (for post-write verification).

Start at the command entry point for npx @microsoft/documentdb-mcp init and review the --validate flag it depends on. The work is done when the wizard detects or asks for a client, merges existing configuration, writes platform-appropriate settings, validates them, prints next steps, and has one unit test per client target.