feat: add UserOpEvent types for audit pipeline

[DeStefaniAndrei]

Summary

Adds UserOpEvent types to the audit crate, following the existing BundleEvent pattern. This is the foundation for tracking UserOperation lifecycle events through the audit pipeline.

Changes

• Add UserOpHash type alias (B256), since in ERC 4337 a UserOpHash is classified as a bytes32 Solidity data type, which translates to B256 in Rust

• Add UserOpDropReason enum with three basic variants:

  • Invalid(String) - validation failure
  • Expired - TTL exceeded
  • ReplacedByHigherFee - replaced by higher fee

• Add UserOpEvent enum with three variants (only events that happen after validation to avoid overloading Kafka):

  • AddedToMempool - when a UserOp passes validation and enters the queue
  • Dropped - when a UserOp is removed from mempool
  • Included - when a UserOp is included in a block

• Add helper methods: user_op_hash() and generate_event_key()

• Add 6 basic unit tests for serialization and accessors

Testing

• cargo test -p tips-audit - 12 tests pass
• cargo clippy -- -D warnings - no warnings
• cargo fmt --all --check - formatted

Planned follow-up PRs

• Wire UserOpEvent::AddedToMempool in ingress-rpc
• Wire UserOpEvent::Included in aa-bundler
• Update Kafka publisher to handle UserOpEvent
• Update S3 archiver for UserOp events

[chunter-cb]

LGTM, as long as we can put in the reverted text then that is standard as the way bundler does it when it drops an OP.

This is going to be a big win compared to visibility