We have recently discovered a workload that hit a MONGO_UNREACHABLE because it was used in instances where the intention was to control unexpected parameters for internal code usage, and we broke that.

Although we have control over how internal code should be used, relying on invariants (MONGO_UNREACHABLE) may not be safe for future usages or unexpected behaviours. Instead, we should consider using `tassert` to prevent potential availability issues in production environments in the worst-case scenario.

The goal of this ticket is to investigate all MONGO_UNREACHABLE usages in the CAR codebase. We should analyze whether we should replace it with `tassert` or if it is appropriate to leave the code as it is.