fix(consensus-db): declare PendingProposalsRepository::count with Self::Error

[Dusk1e]

Every method across the six repository traits in repositories/ and the PruningService trait is declared with Self::Error. PendingProposalsRepository::count is the one exception:

pub trait PendingProposalsRepository {
    type Error: std::error::Error + Send + Sync + 'static;

    async fn enforce_limit(...) -> Result<Vec<(Height, Round, BlockHash)>, Self::Error>;

    async fn count(&self) -> Result<usize, StoreError>;
}

That is not only a style difference. The trait cannot be implemented by anything whose Error is not StoreError — an implementor returning its own error type from count fails to compile:

error[E0271]: expected `impl Future<Output = Result<usize, <FailingCount as PendingProposalsRepository>::Error>>`
to be a future that resolves to `Result<usize, StoreError>`, but it resolves to `Result<usize, CountError>`

So the associated Error type is honoured by one of the trait's two methods, and MockPendingProposalsRepository, generated with Error = std::io::Error, returns io::Error from enforce_limit and StoreError from count.

The blanket impl for &T carried the same hardcoded signature and now forwards Self::Error too. Store is unaffected: its Error already is StoreError. The one caller, consensus_ready.rs, goes through wrap_err, which the trait's existing Error bound already satisfies.

The test is the implementor from the error above: it fails to compile on main and passes with the change. cargo test -p arc-consensus-db is green (107 tests), as is cargo check -p arc-consensus-db --features mock.