testing(`da::celestia`): Create integration tests

`da::celestia` has virtually zero coverage, because it requires either making integration tests or using `mockall` on the RPC interfaces like `BlobClient`.

The solution is probably a combination of both approaches -- some scenarios might be too complicated to reliably reproduce from the celestia environment, but mockall works.

Because we don't own those interfaces, it would require using the `mock!` macro.