💪 [Enhance]: Mongo DB test fixtures to use the `.` separator instead of the forward slash `/`

### Description

The Mongo DB separator is the dot `.` to separate between the Mongo DB database and collections. Because we use the forward slash `/` in the logging, it might seem confusing when this gets translated into real Azure resources during defect localization.

### Solution

Use the dot `.` notation when logging Mongo DB collections in databases to streamline with the naming conventions of Mongo DB.

### Additional context

Test fixtures are located in the `Arcus.Testing.Storage.Cosmos` project.

Start in the Arcus.Testing.Storage.Cosmos project and locate the Mongo DB test fixtures and their logging path. Update the fixture logging to use a dot between the database and collection names, then run the relevant storage tests to verify the logged resource names match Mongo DB conventions.

Written by the indexing model from the issue text.