Fix PSK proxy re-sync: tls_psk_identity overwritten with empty value (#141)

[tomehb]

Fixes #141

Problem

Re-syncing a proxy that uses PSK encryption fails with:

ValidationError: {'tls_psk_identity': ['TLS PSK Identity must be provided when using PSK encryption.']}

Zabbix >= 5.4 does not return tls_psk_identity from proxy.get (it is write-only, the same as tls_psk). ProxySync.sync_from_zabbix() fell back to '' when the key was missing, wiping the identity that had just been pushed to Zabbix. The subsequent save() then failed full_clean() because tls_accept still contained the PSK flag.

This also broke host sync for any host behind a PSK proxy, because SyncHostJob.sync_host() syncs the proxy first and the proxy sync raised before the host was reached.

Changes

nbxsync/utils/sync/proxysync.py: tls_psk_identity now keeps the NetBox value unless Zabbix actually returns one, matching the existing write-only handling of tls_psk.

nbxsync/models/sync_info.py: update_sync_info() now falls back to a direct queryset update if save() raises ValidationError. ZabbixProxy.save() runs full_clean(), so when the sync failed because the object was invalid, the except branch's call to update_sync_info() re-ran full_clean() and raised again. The job died in the RQ failed registry and last_sync_message stayed at "Never synced", which is why the failure in #141 was invisible in the UI. The happy path is unchanged.

Tests

• test_sync_from_zabbix_keeps_tls_psk_identity_when_not_returned: a payload without tls_psk_identity leaves the existing identity untouched.
• test_update_sync_info_persists_when_object_fails_validation: update_sync_info(success=False, message='x') persists the message on a ZabbixProxy whose full_clean() fails.

Both new tests fail on main and pass with this branch. The two affected test modules (25 tests) pass against a NetBox install running Django 6.0.