Follow-up to #6825, which introduced this bug.

We use the `can_build_upon` method to ask the runtime if it is fine to
build another block. The runtime checks this based on the
[`ConsensusHook`](https://github.com/paritytech/polkadot-sdk/blob/c1b7c302/cumulus/pallets/aura-ext/src/consensus_hook.rs#L110-L110)
implementation, the most popular one being the `FixedConsensusHook`.

In #6825 I removed a check that would always allow us to build when we
are building on an included block. Turns out this check is still
required when:
1. The [`UnincludedSegment`
](https://github.com/paritytech/polkadot-sdk/blob/c1b7c302

/cumulus/pallets/parachain-system/src/lib.rs#L758-L758)
storage item in pallet-parachain-system is equal or larger than the
unincluded segment.
2. We are calling the `can_build_upon` runtime API where the included
block has progressed offchain to the current parent block (so last entry
in the `UnincludedSegment` storage item).

In this scenario the last entry in `UnincludedSegment` does not have a
hash assigned yet (because it was not available in `on_finalize` of the
previous block). So the unincluded segment will be reported at its
maximum length which will forbid building another block.

Ideally we would have a more elegant solution than to rely on the
node-side here. But for now the check is reintroduced and a test is
added to not break it again by accident.

---------

Co-authored-by: command-bot <>
Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>