[event] explicitly clear flow info that doesn't apply

Poorly written clients might try to unconditionally read these for
event types they don't apply to, so always clear them. For an
explanation of what these are, see:

> https://docs.monad.xyz/execution-events/event-ring#flow-tags-the-content_ext-fields-in-execution-event-rings

Author: kjcamann

category/execution/ethereum/event/exec_event_recorder.hpp

@@ -290,6 +290,8 @@ inline void ExecutionEventRecorder::record_block_marker_event(
         monad_event_recorder_reserve(&exec_recorder_, 0, &seqno, &payload_buf);
     event->event_type = std::to_underlying(event_type);
     event->content_ext[MONAD_FLOW_BLOCK_SEQNO] = cur_block_start_seqno_;
+    event->content_ext[MONAD_FLOW_TXN_ID] = 0;
+    event->content_ext[MONAD_FLOW_ACCOUNT_INDEX] = 0;
     monad_event_recorder_commit(event, seqno);
 }
 
@@ -303,6 +305,7 @@ inline void ExecutionEventRecorder::record_txn_marker_event(
     event->event_type = std::to_underlying(event_type);
     event->content_ext[MONAD_FLOW_BLOCK_SEQNO] = cur_block_start_seqno_;
     event->content_ext[MONAD_FLOW_TXN_ID] = txn_num + 1;
+    event->content_ext[MONAD_FLOW_ACCOUNT_INDEX] = 0;
     monad_event_recorder_commit(event, seqno);
 }