OP: improve `check_fp_exist()` to also check if the FP is slashed

[maurolacy]

@bap2pecs cloned issue babylonchain/babylon-contract#208 on 2024-07-09:

it's tricky b/c when we query the gRPC we get FinalityProviderResponse which only contain slashed_babylon_height and slashed_btc_height

but we only get the L2 block height in the function argument