Diego fix assethub panics

[jimjbrettj]

Description

Fixes several panics that occurred when forking real Asset Hub (westend) using:

cargo run -p anvil-polkadot -- --fork-url https://westend-asset-hub-rpc.polkadot.io

• Uses the checkpoint block hash directly when creating the OnlineClient in fork mode, instead of trying to derive it from the genesis block number
• Reads the DMQ MQC head from parachain storage (ParachainSystem::LastDmqMqcHead) to properly initialize the MockValidationDataInherentDataProvider, avoiding "DMQ head mismatch" errors during block production
• Adds first integration test for forking from Westend Asset Hub and check the previous changes