[bugfix] Fix imported monitor instance being empty due to host/instan…

[starryCoder]

…ce field mismatch

What's changed?

When importing monitors via JSON/Excel, the instance field (which stores host:port) is always null in the database, because AbstractImExportServiceImpl.convert() uses BeanUtils.copyProperties to map between
MonitorDTO and Monitor, but the field names differ:

• Monitor.instance stores host:port (e.g. localhost:161)
• MonitorDTO.host is used in the export/import JSON format

BeanUtils.copyProperties matches by field name, so instance ↔ host is never mapped.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.