fix(csharp): qualify model return types in generated tests to avoid standard library name collisions

When a model class shares a name with a .NET standard library type (e.g.
`Attributes`), the generated `Assert.IsType<Attributes>(model)` call is
ambiguous and fails to compile. Use the fully-qualified type name
`{{packageName}}.{{modelPackage}}.{{{.}}}` for model return types so the
generated test code is unambiguous regardless of model naming.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Neokil

modules/openapi-generator/src/main/resources/csharp/libraries/generichost/api_test.mustache

@@ -42,7 +42,12 @@ namespace {{packageName}}.Test.{{apiPackage}}
             {{#dataType}}
             var response = await _instance.{{operationId}}Async({{#allParams}}{{paramName}}{{^-last}}, {{/-last}}{{/allParams}});
             var model = response.{{vendorExtensions.x-http-status}}();
+            {{#isModel}}
+            Assert.IsType<{{packageName}}.{{modelPackage}}.{{{.}}}>(model);
+            {{/isModel}}
+            {{^isModel}}
             Assert.IsType<{{{.}}}>(model);
+            {{/isModel}}
             {{/dataType}}
             {{^dataType}}
             await _instance.{{operationId}}Async({{#allParams}}{{paramName}}{{^-last}}, {{/-last}}{{/allParams}});

modules/openapi-generator/src/test/resources/3_0/csharp/model-name-collision.yaml

@@ -0,0 +1,36 @@
+openapi: 3.0.0
+info:
+  title: Model Name Collision Test
+  description: >-
+    Tests that generated test code uses fully-qualified type names for model
+    return types to avoid collisions with standard library classes (e.g. a
+    model named "Attributes" would conflict with System.Attribute).
+  version: 1.0.0
+  license:
+    name: Apache-2.0
+    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
+paths:
+  /resources:
+    get:
+      operationId: GetResources
+      tags:
+        - Resources
+      responses:
+        '200':
+          description: Returns a resource with attributes
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Attributes'
+servers:
+  - url: 'http://localhost'
+components:
+  schemas:
+    Attributes:
+      type: object
+      properties:
+        id:
+          type: integer
+          format: int32
+        name:
+          type: string