merge extensions per key and exclude const/not overlays from the merge

Review feedback. Extensions from both parts now survive, overlay wins
per key. Overlays carrying const or not keep the old replace behavior.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: SubhamAshok

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

@@ -3665,7 +3665,8 @@ private void putProperty(Map<String, Schema> targetProperties, String name, Sche
         Schema existing = targetProperties.get(name);
         if (existing != null && incoming != null
                 && !ModelUtils.isAnyType(existing)
-                && ModelUtils.isMetadataOnlySchema(incoming) && incoming.getEnum() == null) {
+                && ModelUtils.isMetadataOnlySchema(incoming)
+                && incoming.getEnum() == null && incoming.getConst() == null && incoming.getNot() == null) {
             Schema merged = ModelUtils.cloneSchema(existing, specVersionGreaterThanOrEqualTo310(openAPI));
             ModelUtils.copyConstraints(incoming, merged);
             targetProperties.put(name, merged);

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -2771,7 +2771,14 @@ public static void copyMetadata(Schema from, Schema to) {
      * @param to   schema to copy to
      */
     public static void copyConstraints(Schema from, Schema to) {
+        Map<String, Object> targetExtensions = to.getExtensions() == null ? null : new HashMap<>(to.getExtensions());
         copyMetadata(from, to);
+        // merge extensions per key instead of replacing, the source wins on conflicts
+        if (targetExtensions != null && from.getExtensions() != null) {
+            Map<String, Object> mergedExtensions = new HashMap<>(targetExtensions);
+            mergedExtensions.putAll(from.getExtensions());
+            to.setExtensions(mergedExtensions);
+        }
         if (from.getFormat() != null) {
             to.setFormat(from.getFormat());
         }

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

@@ -1330,6 +1330,9 @@ public void testAllOfNullableWithoutTypeKeepsType() {
         assertEquals("String", addressId.dataType);
         assertTrue(addressId.isNullable);
         assertEquals(Integer.valueOf(36), addressId.maxLength);
+        // extensions from both parts survive the merge
+        assertEquals("keep", addressId.vendorExtensions.get("x-base-marker"));
+        assertEquals("added", addressId.vendorExtensions.get("x-overlay-marker"));
     }
 
     @Test

modules/openapi-generator/src/test/resources/3_0/allOf-nullable-typeless-override.yaml

@@ -27,6 +27,7 @@ components:
       properties:
         addressId:
           type: string
+          x-base-marker: keep
     UpdateFirm:
       allOf:
         - $ref: "#/components/schemas/FirmProperties"
@@ -37,3 +38,4 @@ components:
             addressId:
               nullable: true
               maxLength: 36
+              x-overlay-marker: added