feat: Add internal ProblemDetail record for problem+json bodies

Author: thced

src/main/java/com/retailsvc/http/internal/ProblemDetail.java

@@ -0,0 +1,48 @@
+package com.retailsvc.http.internal;
+
+import com.retailsvc.http.BadRequestException;
+import com.retailsvc.http.validate.ValidationError;
+import java.util.Map;
+
+/**
+ * Carrier for an RFC 7807 problem+json document. Serialized by the registered JSON {@code
+ * TypeMapper}; the wire shape and field-order are whatever the configured mapper produces — title
+ * is advisory per RFC 7807 since {@code type} is always {@code about:blank}.
+ */
+public record ProblemDetail(
+    String type, String title, int status, String detail, String pointer, String keyword) {
+
+  private static final String DEFAULT_TYPE = "about:blank";
+
+  public static ProblemDetail forValidation(ValidationError e) {
+    return new ProblemDetail(
+        DEFAULT_TYPE, "Bad Request", 400, e.message(), e.pointer(), e.keyword());
+  }
+
+  public static ProblemDetail forBadRequest(BadRequestException e) {
+    return new ProblemDetail(
+        DEFAULT_TYPE,
+        titleFor(e.status()),
+        e.status(),
+        e.getMessage(),
+        e.pointer().orElse(null),
+        e.keyword().orElse(null));
+  }
+
+  private static final Map<Integer, String> TITLES =
+      Map.of(
+          400, "Bad Request",
+          401, "Unauthorized",
+          403, "Forbidden",
+          404, "Not Found",
+          405, "Method Not Allowed",
+          409, "Conflict",
+          410, "Gone",
+          412, "Precondition Failed",
+          415, "Unsupported Media Type",
+          422, "Unprocessable Content");
+
+  private static String titleFor(int status) {
+    return TITLES.getOrDefault(status, "Bad Request");
+  }
+}