fix(kotlin-spring): use correct Jackson 3 property path for WRITE_DATES_AS_TIMESTAMPS (#23384)

* fix(kotlin-spring): use correct Jackson 3 property path for WRITE_DATES_AS_TIMESTAMPS

When using Spring Boot 4 with Jackson 3, the property path for
WRITE_DATES_AS_TIMESTAMPS changed from:
  spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS
to:
  spring.jackson.datatype.datetime.WRITE_DATES_AS_TIMESTAMPS

This fix adds conditional logic to the kotlin-spring generator template
to use the correct property path based on useSpringBoot4 flag, matching
the behavior already implemented in the JavaSpring generator.

Fixes binding error:
'Failed to bind properties under spring.jackson.serialization to
java.util.Map<tools.jackson.databind.SerializationFeature, java.lang.Boolean>:
No enum constant tools.jackson.databind.SerializationFeature.WRITE_DATES_AS_TIMESTAMPS'

* test(kotlin-spring): add tests for Jackson datetime property path

Add tests to verify that:
- Spring Boot 4 generates application.yaml with spring.jackson.datatype.datetime.WRITE_DATES_AS_TIMESTAMPS
- Spring Boot 3 generates application.yaml with spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS

Author: takkiraz

modules/openapi-generator/src/main/resources/kotlin-spring/libraries/spring-boot/application.mustache

@@ -3,8 +3,15 @@ spring:
     name: {{title}}
 
   jackson:
+{{#useSpringBoot4}}
+    datatype:
+      datetime:
+        WRITE_DATES_AS_TIMESTAMPS: false
+{{/useSpringBoot4}}
+{{^useSpringBoot4}}
     serialization:
       WRITE_DATES_AS_TIMESTAMPS: false
+{{/useSpringBoot4}}
 
 server:
   port: {{serverPort}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/spring/KotlinSpringServerCodegenTest.java

@@ -676,6 +676,53 @@ public void useSpringBoot3() throws Exception {
         );
     }
 
+    @Test(description = "Spring Boot 4 should use Jackson 3 datetime property path")
+    public void useSpringBoot4JacksonDateTimeProperty() throws Exception {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+        String outputPath = output.getAbsolutePath().replace('\\', '/');
+
+        KotlinSpringServerCodegen codegen = new KotlinSpringServerCodegen();
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(KotlinSpringServerCodegen.USE_SPRING_BOOT4, true);
+
+        new DefaultGenerator().opts(new ClientOptInput()
+                        .openAPI(TestUtils.parseSpec("src/test/resources/3_0/petstore.yaml"))
+                        .config(codegen))
+                .generate();
+
+        // Spring Boot 4 uses Jackson 3, which moved WRITE_DATES_AS_TIMESTAMPS to
+        // spring.jackson.datatype.datetime instead of spring.jackson.serialization
+        Path applicationYaml = Paths.get(outputPath + "/src/main/resources/application.yaml");
+        assertFileContains(applicationYaml, "datatype:");
+        assertFileContains(applicationYaml, "datetime:");
+        assertFileContains(applicationYaml, "WRITE_DATES_AS_TIMESTAMPS: false");
+        assertFileNotContains(applicationYaml, "serialization:");
+    }
+
+    @Test(description = "Spring Boot 3 should use Jackson 2 serialization property path")
+    public void useSpringBoot3JacksonSerializationProperty() throws Exception {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+        String outputPath = output.getAbsolutePath().replace('\\', '/');
+
+        KotlinSpringServerCodegen codegen = new KotlinSpringServerCodegen();
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(KotlinSpringServerCodegen.USE_SPRING_BOOT3, true);
+
+        new DefaultGenerator().opts(new ClientOptInput()
+                        .openAPI(TestUtils.parseSpec("src/test/resources/3_0/petstore.yaml"))
+                        .config(codegen))
+                .generate();
+
+        // Spring Boot 3 uses Jackson 2, which has WRITE_DATES_AS_TIMESTAMPS under
+        // spring.jackson.serialization
+        Path applicationYaml = Paths.get(outputPath + "/src/main/resources/application.yaml");
+        assertFileContains(applicationYaml, "serialization:");
+        assertFileContains(applicationYaml, "WRITE_DATES_AS_TIMESTAMPS: false");
+        assertFileNotContains(applicationYaml, "datatype:");
+    }
+
     @Test(description = "multi-line descriptions should be supported for operations")
     public void multiLineOperationDescription() throws IOException {
         testMultiLineOperationDescription(false);

samples/server/petstore/kotlin-springboot-4/src/main/resources/application.yaml

@@ -3,8 +3,9 @@ spring:
     name: openAPIPetstore
 
   jackson:
-    serialization:
-      WRITE_DATES_AS_TIMESTAMPS: false
+    datatype:
+      datetime:
+        WRITE_DATES_AS_TIMESTAMPS: false
 
 server:
   port: 8080