dot/sync: use `runtime.CheckInherents()` in block import process

## Task summary

- use the `runtime.CheckInherents()` function to validate the block inherents before importing a block
- may need clarity from spec team on when exactly this is needed, as substrate seems to sometimes not use it?

## Other information and links

https://github.com/paritytech/substrate/blob/4c3a55e7ca5c4c85c1eb53fd82ed71029d952510/primitives/inherents/src/lib.rs#L38

Start at the block import path and the runtime.CheckInherents() API, then compare the linked Substrate inherents implementation. Confirm with the spec team when validation is required; done means inherents are checked before a block is imported according to the agreed rule.

Written by the indexing model from the issue text.