feat: Preserve OpenAPI extensions on every Schema record

Add Map<String, Object> extensions as the final component to all 16
concrete Schema records and as an abstract method on the sealed Schema
interface. SchemaParser.extractExtensions() threads x-* keys from the
raw map through every concrete constructor; synthesized schemas with
no source map receive Map.of(). DefaultValidator's record-deconstruct
patterns gain the extra var _ binding. All existing test call sites
updated with Map.of() as the new last argument. Three new tests in
ExtensionsTest verify ObjectSchema, StringSchema, and OneOfSchema
extension exposure.

Author: thced

src/main/java/com/retailsvc/http/spec/schema/AllOfSchema.java

@@ -1,9 +1,10 @@
 package com.retailsvc.http.spec.schema;
 
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
-public record AllOfSchema(List<Schema> parts) implements Schema {
+public record AllOfSchema(List<Schema> parts, Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();

src/main/java/com/retailsvc/http/spec/schema/AlwaysSchema.java

@@ -1,8 +1,9 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
-public record AlwaysSchema() implements Schema {
+public record AlwaysSchema(Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();

src/main/java/com/retailsvc/http/spec/schema/AnyOfSchema.java

@@ -1,9 +1,10 @@
 package com.retailsvc.http.spec.schema;
 
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
-public record AnyOfSchema(List<Schema> options) implements Schema {
+public record AnyOfSchema(List<Schema> options, Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();

src/main/java/com/retailsvc/http/spec/schema/ArraySchema.java

@@ -1,7 +1,13 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
 public record ArraySchema(
-    Set<TypeName> types, Schema items, Integer minItems, Integer maxItems, boolean uniqueItems)
+    Set<TypeName> types,
+    Schema items,
+    Integer minItems,
+    Integer maxItems,
+    boolean uniqueItems,
+    Map<String, Object> extensions)
     implements Schema {}

src/main/java/com/retailsvc/http/spec/schema/BooleanSchema.java

@@ -1,5 +1,7 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
-public record BooleanSchema(Set<TypeName> types) implements Schema {}
+public record BooleanSchema(Set<TypeName> types, Map<String, Object> extensions)
+    implements Schema {}

src/main/java/com/retailsvc/http/spec/schema/ConstSchema.java

@@ -1,8 +1,9 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
-public record ConstSchema(Object value) implements Schema {
+public record ConstSchema(Object value, Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();

src/main/java/com/retailsvc/http/spec/schema/EnumSchema.java

@@ -1,9 +1,10 @@
 package com.retailsvc.http.spec.schema;
 
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
-public record EnumSchema(List<Object> values) implements Schema {
+public record EnumSchema(List<Object> values, Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();

src/main/java/com/retailsvc/http/spec/schema/IntegerSchema.java

@@ -1,5 +1,6 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
 public record IntegerSchema(
@@ -9,5 +10,6 @@ public record IntegerSchema(
     Long exclusiveMinimum,
     Long exclusiveMaximum,
     Long multipleOf,
-    String format)
+    String format,
+    Map<String, Object> extensions)
     implements Schema {}

src/main/java/com/retailsvc/http/spec/schema/NeverSchema.java

@@ -1,8 +1,9 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
-public record NeverSchema() implements Schema {
+public record NeverSchema(Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();

src/main/java/com/retailsvc/http/spec/schema/NotSchema.java

@@ -1,8 +1,9 @@
 package com.retailsvc.http.spec.schema;
 
+import java.util.Map;
 import java.util.Set;
 
-public record NotSchema(Schema schema) implements Schema {
+public record NotSchema(Schema schema, Map<String, Object> extensions) implements Schema {
   @Override
   public Set<TypeName> types() {
     return Set.of();