.Net: Fix MongoDB test container ready detection

[damieng]

Motivation and Context

To fix unreliable MongoDB Test Container start-up.

Fixes #13025

Description

TestContainers.NET is, by default checking, for the string Waiting for connections to appear TWICE in the MongoDB logs before deciding the container is available. While this can happen quite often it doesn't always happen and I believe this is a mistake (introduced in a large PR).

TestContainers clients for Go and Rust only check the message appears once (and TestContainers for Java has a different mechanism altogether).

This introduces an alternative waiting mechanism until I can get the fix shipped upstream that only checks the message appears once.

Note: This PR does not yet enable the MongoDB tests as I think there are some other intermittent issues relating to the use of lock/counting mechanism that sometimes responds with a very different error.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄