fix(typescript-fetch): skip scalar enum check for array-typed properties in instanceOf (#24217)

The instanceOf guard was emitting an invalid scalar !== comparison for
  required properties typed as Array<SingleValueEnum>, introduced as a
  side effect of PR #23497. The enum-literal check block now only fires
  for non-container (scalar) properties

Author: mstojanovic8

modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

@@ -32,6 +32,7 @@ export function instanceOf{{classname}}(value: object): value is {{classname}} {
     if (!('{{name}}' in value) || value['{{name}}'] === undefined) return false;
     {{/hasSanitizedName}}
     {{#isEnum}}
+    {{^isContainer}}
     {{#allowableValues}}
     {{#values}}
     {{#-first}}
@@ -48,6 +49,7 @@ export function instanceOf{{classname}}(value: object): value is {{classname}} {
     {{/-first}}
     {{/values}}
     {{/allowableValues}}
+    {{/isContainer}}
     {{/isEnum}}
     {{/required}}
     {{/vars}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/fetch/TypeScriptFetchClientCodegenTest.java

@@ -653,6 +653,21 @@ public void testNestedOneOfGeneratesTypeAliasForOneOfParent() throws IOException
         TestUtils.assertFileContains(outerPlain, "export interface OuterPlain {");
     }
 
+    @Test(description = "instanceOf guard must not emit scalar enum comparison for array-typed enum properties")
+    public void testInstanceOfArrayEnumNoScalarComparison() throws Exception {
+        File output = generate(
+            Collections.emptyMap(),
+            "src/test/resources/3_0/typescript-fetch/array_of_single_value_enum.json"
+        );
+
+        Path modelPath = Paths.get(output + "/models/TestSchema.ts");
+        // Must NOT emit a scalar !== comparison for an array-typed enum property
+        TestUtils.assertFileNotContains(modelPath, "value['types'] !== 'boatbooker_activities'");
+        TestUtils.assertFileNotContains(modelPath, "value['types'] !== boatbooker_activities");
+        // Must still check for presence of the field
+        TestUtils.assertFileContains(modelPath, "'types' in value");
+    }
+
     @Test(description = "Optional nullable fields should deserialize to null, not undefined (fix #5670)")
     public void testOptionalNullableFieldDeserializesToNull() throws Exception {
         File output = generate(

modules/openapi-generator/src/test/resources/3_0/typescript-fetch/array_of_single_value_enum.json

@@ -0,0 +1,25 @@
+{
+  "openapi": "3.0.1",
+  "info": {
+    "title": "Array of Single-Value Enum Test",
+    "version": "1.0.0"
+  },
+  "paths": {},
+  "components": {
+    "schemas": {
+      "TestSchema": {
+        "required": ["types"],
+        "type": "object",
+        "properties": {
+          "types": {
+            "type": "array",
+            "items": {
+              "type": "string",
+              "enum": ["boatbooker_activities"]
+            }
+          }
+        }
+      }
+    }
+  }
+}