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

docs/configuration/data-validation.md

@@ -8,7 +8,7 @@ This is a mapping of models to [custom validators](../customization/custom-valid
 
 ```python
 CUSTOM_VALIDATORS = {
-    "dcim.site": [
+    "dcim.Site": [
         {
             "name": {
                 "min_length": 5,
@@ -17,12 +17,15 @@ CUSTOM_VALIDATORS = {
         },
         "my_plugin.validators.Validator1"
     ],
-    "dcim.device": [
+    "dcim.Device": [
         "my_plugin.validators.Validator1"
     ]
 }
 ```
 
+!!! info "Case-Insensitive Model Names"
+    Model identifiers are case-insensitive. Both `dcim.site` and `dcim.Site` are valid and equivalent.
+
 ---
 
 ## FIELD_CHOICES
@@ -53,6 +56,9 @@ FIELD_CHOICES = {
 }
 ```
 
+!!! info "Case-Insensitive Field Identifiers"
+    Field identifiers are case-insensitive. Both `dcim.Site.status` and `dcim.site.status` are valid and equivalent.
+
 The following model fields support configurable choices:
 
 * `circuits.Circuit.status`
@@ -98,7 +104,7 @@ This is a mapping of models to [custom validators](../customization/custom-valid
 
 ```python
 PROTECTION_RULES = {
-    "dcim.site": [
+    "dcim.Site": [
         {
             "status": {
                 "eq": "decommissioning"
@@ -108,3 +114,6 @@ PROTECTION_RULES = {
     ]
 }
 ```
+
+!!! info "Case-Insensitive Model Names"
+    Model identifiers are case-insensitive. Both `dcim.site` and `dcim.Site` are valid and equivalent.