fix: use ConversionError instead of unit type in receipt conversion

[oooLowNeoNooo]

Problem

The TryFrom<AnyTransactionReceipt> implementation for FoundryTxReceipt uses () as the error type, which provides no information about conversion failures. When an unknown transaction receipt type is encountered, callers receive an uninformative error that makes debugging difficult.

Solution

Replace the unit error type with ConversionError from alloy_rpc_types, matching the pattern used in TryFrom<AnyRpcTransaction>. The error now includes the unknown transaction type in the message (e.g., "Unknown transaction receipt type: 0x05"), improving error diagnostics and maintainability.