[Go] Honor generateUnmarshalJSON=false for oneOf models (OpenAPITools#24062)

The oneOf model template always emitted UnmarshalJSON (which calls
validator.Validate), but the gopkg.in/validator.v2 import is only added
when generateUnmarshalJSON is true. With generateUnmarshalJSON=false the
generated oneOf code therefore referenced an import that was no longer
present and failed to compile.

Wrap the oneOf UnmarshalJSON in the x-go-generate-unmarshal-json vendor
extension, mirroring model_simple.mustache, so the flag consistently
controls UnmarshalJSON generation for oneOf models. With the default
(true) the generated output is unchanged.

Fixes OpenAPITools#24053

Author: seonwooj0810

modules/openapi-generator/src/main/resources/go/model_oneof.mustache

@@ -15,6 +15,7 @@ func {{#lambda.type-to-name}}{{{.}}}{{/lambda.type-to-name}}As{{classname}}(v *{
 
 {{/oneOf}}
 
+{{#vendorExtensions.x-go-generate-unmarshal-json}}
 // Unmarshal JSON data into one of the pointers in the struct
 func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	var err error
@@ -119,6 +120,7 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	{{/useOneOfDiscriminatorLookup}}
 }
 
+{{/vendorExtensions.x-go-generate-unmarshal-json}}
 // Marshal data from the first non-nil pointers in the struct to JSON
 func (src {{classname}}) MarshalJSON() ([]byte, error) {
 {{#oneOf}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/go/GoClientCodegenTest.java

@@ -515,4 +515,51 @@ public void testEscapingInExamples() throws IOException {
         // Verify that quotes are properly escaped in email parameter examples
         TestUtils.assertFileContains(docPath, "emailWithQuotes := \"test\\\"user@example.com\"");
     }
+
+    @Test(description = "generateUnmarshalJSON=false must also suppress the oneOf UnmarshalJSON so the generated code does not reference the validator import that is no longer added (#24053)")
+    public void testOneOfUnmarshalJSONHonorsGenerateUnmarshalJSONFlag() throws IOException {
+        Map<String, Object> properties = new HashMap<>();
+        properties.put(CodegenConstants.GENERATE_UNMARSHAL_JSON, false);
+
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("go")
+                .setAdditionalProperties(properties)
+                .setInputSpec("src/test/resources/3_0/go/spec-with-oneof-anyof-required.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        // With the flag disabled the validator import is not added, so the oneOf model must not
+        // emit UnmarshalJSON (which would reference the missing validator package and fail to compile).
+        Path oneOfModel = Paths.get(output + "/model_object.go");
+        TestUtils.assertFileNotContains(oneOfModel, "func (dst *Object) UnmarshalJSON");
+        TestUtils.assertFileNotContains(oneOfModel, "validator.Validate");
+        TestUtils.assertFileNotContains(oneOfModel, "gopkg.in/validator.v2");
+    }
+
+    @Test(description = "with the default generateUnmarshalJSON=true the oneOf UnmarshalJSON and the validator import are still generated")
+    public void testOneOfUnmarshalJSONGeneratedByDefault() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("go")
+                .setInputSpec("src/test/resources/3_0/go/spec-with-oneof-anyof-required.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        Path oneOfModel = Paths.get(output + "/model_object.go");
+        TestUtils.assertFileContains(oneOfModel,
+                "func (dst *Object) UnmarshalJSON",
+                "validator.Validate",
+                "gopkg.in/validator.v2");
+    }
 }