bugfix: fix network inbound queue delay underflow

[Lchangliang]

Summary

• Guard inbound network queue delay calculation against reversed wall-clock timestamps
• Log a warning and record 0 seconds when dequeue time is earlier than receive time
• Add unit coverage for normal and reversed timestamp calculations

Root cause

ReceivedMessage::new() records receive_timestamp_micros using Unix wall-clock time, and received_message_to_event() later reads Unix wall-clock time again before observing inbound queue delay. If the system clock moves backwards between those two reads, or a manually constructed ReceivedMessage carries a future receive timestamp, dequeue_at - rx_at underflows and can panic.

Validation

• cargo fmt --package aptos-network
• git diff --check
• cargo test -p aptos-network inbound_queue_delay_seconds