Mobile container inventory UI gets doubled.

When a player holds a mobile container like a toolbox or medkit, opening it can show twice as many slots as the container actually has.

Expected behavior:
- The same mobile container should not be counted twice.
- The inventory UI should show each available container only once.

--- Original GitHub issue (auto-fetched) ---
Title: Resolved doubling of Mobile Container UI

Body:
### Summary

Removes the issue where twice the number of slots appeared for held toolboxes / medkits.

## Technical Notes (optional)

AddRange() was being used to add Containers within the Inventory.GetContainers() method. This led to the held mobile inventories being returned by both twice (once in their own right, once as a child of the player inventory). Simple check added to prevent duplicate addition into list.

## Fixes

Closes #514
