feat(rust): MongoDB adapter (connect + list/find) #1893

Parent

#1850

Goal

Minimal Mongo: connect, list collections, find with limit.

Done when

• Same testing bar

How to work

• Branch: issue/<n>-slug from epic/tauri-migration (create epic branch if missing).
• One PR → Closes #<n>.
• Target PR base: epic/tauri-migration until the SQLite slice is green.
• Do not edit Flutter app code unless this issue says so.