Closes #21209: Accept case-insensitive model names in configuration (#21275)

NetBox now accepts case-insensitive model identifiers in configuration, allowing
both lowercase (e.g. "dcim.site") and PascalCase (e.g. "dcim.Site") for
DEFAULT_DASHBOARD, CUSTOM_VALIDATORS, and PROTECTION_RULES.
This makes model name handling consistent with FIELD_CHOICES.

- Add a shared case-insensitive config lookup helper (get_config_value_ci())
- Use the helper in extras/signals.py and core/signals.py
- Update FIELD_CHOICES ChoiceSetMeta to support case-insensitive replace/extend
  (only compute extend choices if no replacement is defined)
- Add unit tests for get_config_value_ci()
- Add integration tests for case-insensitive FIELD_CHOICES replacement/extension
- Update documentation examples to use PascalCase consistently

netbox/core/signals.py

@@ -18,6 +18,7 @@ from extras.events import enqueue_event
 from extras.models import Tag
 from extras.utils import run_validators
 from netbox.config import get_config
+from utilities.data import get_config_value_ci
 from netbox.context import current_request, events_queue
 from netbox.models.features import ChangeLoggingMixin, get_model_features, model_is_public
 from utilities.exceptions import AbortRequest
@@ -168,7 +169,7 @@ def handle_deleted_object(sender, instance, **kwargs):
     # to queueing any events for the object being deleted, in case a validation error is
     # raised, causing the deletion to fail.
     model_name = f'{sender._meta.app_label}.{sender._meta.model_name}'
-    validators = get_config().PROTECTION_RULES.get(model_name, [])
+    validators = get_config_value_ci(get_config().PROTECTION_RULES, model_name, default=[])
     try:
         run_validators(instance, validators)
     except ValidationError as e: