fix: emit : long for int64 enum declarations and add test assertions

The enum declaration template now checks `{{#isLong}}` to emit `: long`
as the underlying type, preventing overflow for int64 enum values.

Added test assertions verifying:
- LongEnum has `: long` in its declaration
- IntegerEnum does NOT have `: long` or `: byte`

Co-authored-by: alexaka1 <22166651+alexaka1@users.noreply.github.com>

Author: Copilot

modules/openapi-generator/src/main/resources/csharp/modelEnum.mustache

@@ -21,7 +21,7 @@
     {{/-first}}
     {{/enumVars}}
     {{/allowableValues}}
-    {{>visibility}} enum {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{#vendorExtensions.x-enum-byte}}: byte{{/vendorExtensions.x-enum-byte}}
+    {{>visibility}} enum {{datatypeWithEnum}}{{^datatypeWithEnum}}{{classname}}{{/datatypeWithEnum}}{{#vendorExtensions.x-enum-byte}}: byte{{/vendorExtensions.x-enum-byte}}{{#isLong}}: long{{/isLong}}
     {
         {{#allowableValues}}
         {{#enumVars}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/csharpnetcore/CSharpClientCodegenTest.java

@@ -293,7 +293,9 @@ public void testIntegerEnumJsonConverterUsesNumericOperations() throws IOExcepti
         );
         assertFileNotContains(intEnumFile.toPath(),
                 "reader.GetString()",
-                "writer.WriteStringValue("
+                "writer.WriteStringValue(",
+                ": long",
+                ": byte"
         );
 
         // Verify long enum uses int64 reader with validation and actual int64 values
@@ -303,6 +305,7 @@ public void testIntegerEnumJsonConverterUsesNumericOperations() throws IOExcepti
         );
         assertNotNull(longEnumFile, "Could not find file for model: LongEnum");
         assertFileContains(longEnumFile.toPath(),
+                "enum LongEnum: long",
                 "reader.GetInt64().ToString()",
                 "FromStringOrDefault(rawValue)",
                 "throw new JsonException()",