FIX: QOL: non fatal utf8 handling.

[braindigitalis]

The user can't catch these errors. We should be much more forgiving on what we accept because UTF8 is a total nightmare for end users. If there is something really bad we should let discord API report it as we do for other stuff. nlohmann will now replace invalid utf8 with � instead of outright throwing and reconnecting the shard. This is much more durable, stable, and user friendly.

Code change checklist

• [x] I have ensured that all methods and functions are fully documented using doxygen style comments.
• [x] My code follows the coding style guide.
• [x] I tested that my change works before raising the PR.
• [x] I have ensured that I did not break any existing API calls.
• [x] I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

I have added a unit test for this.

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