feat: Add branch errors to ValidationError

Author: thced

src/main/java/com/retailsvc/http/validate/ValidationError.java

@@ -1,4 +1,20 @@
 package com.retailsvc.http.validate;
 
+import java.util.List;
+
 public record ValidationError(
-    String pointer, String keyword, String message, Object rejectedValue) {}
+    String pointer,
+    String keyword,
+    String message,
+    Object rejectedValue,
+    List<ValidationError> branches) {
+
+  public ValidationError {
+    branches = List.copyOf(branches);
+  }
+
+  /** Leaf error with no branch sub-errors. */
+  public ValidationError(String pointer, String keyword, String message, Object rejectedValue) {
+    this(pointer, keyword, message, rejectedValue, List.of());
+  }
+}

src/test/java/com/retailsvc/http/validate/ValidationErrorTest.java

@@ -0,0 +1,27 @@
+package com.retailsvc.http.validate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class ValidationErrorTest {
+
+  @Test
+  void convenienceConstructorDefaultsToNoBranches() {
+    var e = new ValidationError("/x", "type", "expected string", null);
+    assertThat(e.branches()).isEmpty();
+  }
+
+  @Test
+  void canonicalConstructorCopiesBranchesDefensively() {
+    var branch = new ValidationError("/x/y", "type", "expected number", "s");
+    var mutable = new ArrayList<ValidationError>(List.of(branch));
+
+    var e = new ValidationError("/x", "oneOf", "matched 0 of 1 oneOf branches", "s", mutable);
+    mutable.clear();
+
+    assertThat(e.branches()).containsExactly(branch);
+  }
+}