feat(dcim): Add device, module and rack count filters

Introduces `device_count`, `module_count` and `rack_count` filters to
enable queries based on the existence and count of the associated
device, module or rack instances.
Updates forms, filtersets, and GraphQL schema to support these filters,
along with tests for validation.

Fixes #19523

netbox/dcim/models/racks.py

@@ -19,9 +19,11 @@ from netbox.models.mixins import WeightMixin
 from netbox.models.features import ContactsMixin, ImageAttachmentsMixin
 from utilities.conversion import to_grams
 from utilities.data import array_to_string, drange
-from utilities.fields import ColorField
+from utilities.fields import ColorField, CounterCacheField
+from utilities.tracking import TrackingModelMixin
 from .device_components import PowerPort
-from .devices import Device, Module
+from .devices import Device
+from .modules import Module
 from .power import PowerFeed
 
 __all__ = (
@@ -144,6 +146,10 @@ class RackType(RackBase):
         max_length=100,
         unique=True
     )
+    rack_count = CounterCacheField(
+        to_model='dcim.Rack',
+        to_field='rack_type'
+    )
 
     clone_fields = (
         'manufacturer', 'form_factor', 'width', 'u_height', 'desc_units', 'outer_width', 'outer_height', 'outer_depth',
@@ -234,7 +240,7 @@ class RackRole(OrganizationalModel):
         verbose_name_plural = _('rack roles')
 
 
-class Rack(ContactsMixin, ImageAttachmentsMixin, RackBase):
+class Rack(ContactsMixin, ImageAttachmentsMixin, TrackingModelMixin, RackBase):
     """
     Devices are housed within Racks. Each rack has a defined height measured in rack units, and a front and rear face.
     Each Rack is assigned to a Site and (optionally) a Location.
@@ -509,7 +515,7 @@ class Rack(ContactsMixin, ImageAttachmentsMixin, RackBase):
 
         return [u for u in elevation.values()]
 
-    def get_available_units(self, u_height=1, rack_face=None, exclude=None, ignore_excluded_devices=False):
+    def get_available_units(self, u_height=1.0, rack_face=None, exclude=None, ignore_excluded_devices=False):
         """
         Return a list of units within the rack available to accommodate a device of a given U height (default 1).
         Optionally exclude one or more devices when calculating empty units (needed when moving a device from one
@@ -581,9 +587,10 @@ class Rack(ContactsMixin, ImageAttachmentsMixin, RackBase):
         :param unit_height: Height of each rack unit for the rendered drawing. Note this is not the total
             height of the elevation
         :param legend_width: Width of the unit legend, in pixels
-        :param margin_width: Width of the rigth-hand margin, in pixels
+        :param margin_width: Width of the right-hand margin, in pixels
         :param include_images: Embed front/rear device images where available
         :param base_url: Base URL for links and images. If none, URLs will be relative.
+        :param highlight_params: Dictionary of parameters to be passed to the RackElevationSVG.render_highlight() method
         """
         elevation = RackElevationSVG(
             self,