fix(circuits): Clear Circuit Termination cache on change

Move cache update logic from signal to model save method and track
original values to properly clear old cache when circuit_id or term_side
changes. Add comprehensive tests for all cache update scenarios.

Fixes #21686

netbox/circuits/models/circuits.py

@@ -347,6 +347,13 @@ class CircuitTermination(
         verbose_name = _('circuit termination')
         verbose_name_plural = _('circuit terminations')
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        # Cache original values to detect changes
+        self._orig_circuit_id = self.__dict__.get('circuit_id')
+        self._orig_term_side = self.__dict__.get('term_side')
+
     def __str__(self):
         return f'{self.circuit}: Termination {self.term_side}'
 
@@ -360,11 +367,39 @@ class CircuitTermination(
             raise ValidationError(_("A circuit termination must attach to a terminating object."))
 
     def save(self, *args, **kwargs):
+        is_new = self._state.adding
+        update_fields = kwargs.get('update_fields')
+
+        # Only consider circuit/term_side changes if those fields
+        # are actually being persisted
+        if update_fields is not None:
+            tracking_relevant = 'circuit' in update_fields or 'term_side' in update_fields
+        else:
+            tracking_relevant = True
+
+        circuit_changed = tracking_relevant and self._orig_circuit_id and self._orig_circuit_id != self.circuit_id
+        term_side_changed = tracking_relevant and self._orig_term_side and self._orig_term_side != self.term_side
+
         # Cache objects associated with the terminating object (for filtering)
         self.cache_related_objects()
 
         super().save(*args, **kwargs)
 
+        # Clear the old termination reference if circuit or term_side changed
+        if circuit_changed or term_side_changed:
+            old_termination_name = f'termination_{self._orig_term_side.lower()}'
+            Circuit.objects.filter(pk=self._orig_circuit_id).update(**{old_termination_name: None})
+
+        # Update the cache if this is a new termination or circuit/term_side changed
+        if is_new or circuit_changed or term_side_changed:
+            # Update the new circuit's termination reference
+            termination_name = f'termination_{self.term_side.lower()}'
+            Circuit.objects.filter(pk=self.circuit_id).update(**{termination_name: self.pk})
+
+            # Update cached values for subsequent saves
+            self._orig_circuit_id = self.circuit_id
+            self._orig_term_side = self.term_side
+
     def cache_related_objects(self):
         self._provider_network = self._region = self._site_group = self._site = self._location = None
         if self.termination_type: