fix(go): do not generate unused imports (#22524)

Fixes #20599

Author: thiagoarrais

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/GoClientCodegen.java

@@ -520,7 +520,7 @@ public ModelsMap postProcessModels(ModelsMap objs) {
                 addedFmtImport = true;
             }
 
-            if (model.hasRequired) {
+            if (generateUnmarshalJSON && model.hasRequired) {
                 if (!model.isAdditionalPropertiesTrue &&
                         (model.oneOf == null || model.oneOf.isEmpty()) &&
                         (model.anyOf == null || model.anyOf.isEmpty())) {

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

@@ -354,6 +354,27 @@ public void testVendorExtensionSkipGenerateUnmarshalJson() throws IOException {
                 "func (o *BaseItem) UnmarshalJSON(data []byte) (err error) {");
     }
 
+    @Test
+    public void testNoImportsWithoutUnmarshal() 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/petstore.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        TestUtils.assertFileNotContains(Paths.get(output + "/model_pet.go"), "bytes");
+    }
+
     @Test
     public void testAdditionalPropertiesWithGoMod() throws Exception {
         File output = Files.createTempDirectory("test").toFile();