fix(templatetags): render hostgroup/tag previews with representative device

[berlikm]

Problem

Templates like Roles/{{ object.role.name }} (assigned to DeviceRole) and Sites/{{ object.site.group.name }}/{{ object.site.name }} (assigned to SiteGroup) fail in the NetBox UI 'Rendered Value' preview card and ObjectViewTable. The render context uses the assignment target (DeviceRole/SiteGroup) as object, which lacks .role/.site:

Undefined variable in template 'Roles/{{ object.role.name }}': 'dcim.models.devices.DeviceRole object' has no attribute 'role'

The sync engine was already fixed in #110 to pass object=Device explicitly, but the UI preview path (render_zabbix_hostgroup_assignment / render_zabbix_tag_assignment) was never addressed and leaked the raw Jinja2 UndefinedError string into the page.

Fix

Substitute a representative Device/VM that inherits the assignment so device-context templates resolve to the same value the sync produces. When no representative is found (empty role/site), the tag returns '' instead of the error string.

Files

• nbxsync/utils/preview.py (new): get_representative_device(assignment) resolver with dispatch table covering DeviceRole, Platform, Manufacturer, DeviceType, Site, SiteGroup (recursive via get_descendants), Region (recursive), Cluster, ClusterType. Cached via @lru_cache(maxsize=64) for the table case.
• nbxsync/templatetags/zabbix_hostgroups.py: takes_context=True, substitute representative when no explicit object= passed. if 'object' not in extra guard preserves the sync-engine path (which passes object=Device explicitly).
• nbxsync/templatetags/zabbix_tags.py: mirror for tags.
• nbxsync/tests/templatetags/test_preview_render.py (+ __init__.py): covers DeviceRole rep, recursive SiteGroup, empty role (graceful ''), static passthrough, explicit object= override.

Behaviour

Template	Before	After
Roles/{{ object.role.name }} on DeviceRole 'Cohesity'	Error string	Roles/Cohesity
Sites/{{ object.site.group.name }}/{{ object.site.name }} on SiteGroup 'CH'	Error string	Sites/CH-NKN/CH-NKN-G08 (first CH device)
Managed/nbxSync (static)	Managed/nbxSync	Managed/nbxSync (unchanged)
Empty role with no devices	Error string	'' (graceful)

Compatibility

• Sync engine unaffected: HostSync already passes object=Device explicitly (Fix: Pass device context to tag render for Jinja2 values #110), and the if 'object' not in extra guard keeps the resolver dormant on that path.
• No model changes, no migrations.
• No dependencies on other open PRs; does not touch any files modified by Fix: Pass device context to tag render for Jinja2 values #110–feat: add use_oob_ip field for vendor-agnostic OOB IP resolution #129.

Supplements #102 (taofineberg's related-context PR) — #102 adds alternative context keys (device, site, role); this PR overrides object so existing templates keep working without rewrite.

[light-foot]

This is a much needed PR. It makes things more NetBox native.