fix: Reject null branches in ValidationError with a clear message

Author: thced

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

@@ -1,6 +1,7 @@
 package com.retailsvc.http.validate;
 
 import java.util.List;
+import java.util.Objects;
 
 public record ValidationError(
     String pointer,
@@ -10,6 +11,8 @@ public record ValidationError(
     List<ValidationError> branches) {
 
   public ValidationError {
+    Objects.requireNonNull(
+        branches, "branches must not be null; use the 4-arg constructor for a leaf error");
     branches = List.copyOf(branches);
   }
 

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

@@ -1,6 +1,7 @@
 package com.retailsvc.http.validate;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatNullPointerException;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -24,4 +25,11 @@ void canonicalConstructorCopiesBranchesDefensively() {
 
     assertThat(e.branches()).containsExactly(branch);
   }
+
+  @Test
+  void nullBranchesArgumentThrows() {
+    assertThatNullPointerException()
+        .isThrownBy(() -> new ValidationError("/x", "oneOf", "summary", "s", null))
+        .withMessageContaining("branches");
+  }
 }