Add comprehensive test cases for this fix.

Author: k2tzumi

test_specs/nullable_enum.yaml

@@ -0,0 +1,131 @@
+openapi: 3.0.2
+info:
+  title: Nullable Enum Test API
+  version: 1.0.0
+  description: Test specification for nullable enum validation
+paths:
+  /status:
+    get:
+      summary: Get status with nullable enum
+      operationId: getStatus
+      responses:
+        '200':
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/StatusResponse'
+    post:
+      summary: Create status
+      operationId: createStatus
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/StatusRequest'
+      responses:
+        '201':
+          description: Created
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/StatusResponse'
+  /items:
+    get:
+      summary: Get items with nullable enum in array
+      operationId: getItems
+      responses:
+        '200':
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Item'
+components:
+  schemas:
+    StatusResponse:
+      type: object
+      required:
+        - id
+      properties:
+        id:
+          type: integer
+          format: int64
+        status:
+          type: string
+          description: Status field with nullable enum (no null in enum)
+          enum:
+            - active
+            - inactive
+            - pending
+            - archived
+          nullable: true
+        priority:
+          type: string
+          description: Priority field with nullable enum (null already in enum)
+          enum:
+            - high
+            - medium
+            - low
+            - null
+          nullable: true
+        category:
+          type: string
+          description: Non-nullable enum
+          enum:
+            - public
+            - private
+            - internal
+    StatusRequest:
+      type: object
+      required:
+        - status
+      properties:
+        status:
+          type: string
+          enum:
+            - active
+            - inactive
+            - pending
+            - archived
+          nullable: true
+        priority:
+          type: string
+          enum:
+            - high
+            - medium
+            - low
+            - null
+          nullable: true
+    Item:
+      type: object
+      required:
+        - id
+        - name
+      properties:
+        id:
+          type: integer
+          format: int64
+        name:
+          type: string
+        status:
+          type: string
+          description: Nested nullable enum
+          enum:
+            - available
+            - sold
+            - reserved
+          nullable: true
+        metadata:
+          type: object
+          properties:
+            visibility:
+              type: string
+              description: Deeply nested nullable enum
+              enum:
+                - visible
+                - hidden
+              nullable: true