fix: add space before colon in enum type declaration per C# convention

Changes `enum Name: long` to `enum Name : long` (and same for `: byte`)
to match standard C# formatting.

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}}{{#isLong}}: long{{/isLong}}
+    {{>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

@@ -305,7 +305,7 @@ public void testIntegerEnumJsonConverterUsesNumericOperations() throws IOExcepti
         );
         assertNotNull(longEnumFile, "Could not find file for model: LongEnum");
         assertFileContains(longEnumFile.toPath(),
-                "enum LongEnum: long",
+                "enum LongEnum : long",
                 "reader.GetInt64().ToString()",
                 "FromStringOrDefault(rawValue)",
                 "throw new JsonException()",