refactor(internal): Extract ProblemDetailRenderer helpers and constants

Splits render() into per-field appenders, names magic strings/status as
constants (PROBLEM_TYPE/TITLE/STATUS), pulls control-char threshold to
FIRST_PRINTABLE_ASCII, adds Javadoc on the unicode-escape path.

Author: thced

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

@@ -2,42 +2,75 @@
 
 import com.retailsvc.http.validate.ValidationError;
 
+/**
+ * Renders a {@link ValidationError} as an RFC 7807 {@code application/problem+json} document.
+ *
+ * <p>Hand-rolled to avoid pulling in a JSON library; only six fixed fields are emitted, all with
+ * known shapes, so a writer-from-scratch is safer than a generic encoder.
+ */
 public final class ProblemDetailRenderer {
+
+  private static final String PROBLEM_TYPE = "about:blank";
+  private static final String PROBLEM_TITLE = "Bad Request";
+  private static final int PROBLEM_STATUS = 400;
+
+  /** Codepoints below this value are control characters and must be unicode-escaped in JSON. */
+  private static final int FIRST_PRINTABLE_ASCII = 0x20;
+
   private ProblemDetailRenderer() {}
 
   public static String render(ValidationError error) {
-    return "{"
-        + "\"type\":\"about:blank\","
-        + "\"title\":\"Bad Request\","
-        + "\"status\":400,"
-        + "\"detail\":\""
-        + escape(error.message())
-        + "\","
-        + "\"pointer\":\""
-        + escape(error.pointer())
-        + "\","
-        + "\"keyword\":\""
-        + escape(error.keyword())
-        + "\""
-        + "}";
+    StringBuilder out = new StringBuilder(128);
+    out.append('{');
+    appendStringField(out, "type", PROBLEM_TYPE);
+    out.append(',');
+    appendStringField(out, "title", PROBLEM_TITLE);
+    out.append(',');
+    appendIntField(out, "status", PROBLEM_STATUS);
+    out.append(',');
+    appendStringField(out, "detail", error.message());
+    out.append(',');
+    appendStringField(out, "pointer", error.pointer());
+    out.append(',');
+    appendStringField(out, "keyword", error.keyword());
+    out.append('}');
+    return out.toString();
+  }
+
+  private static void appendStringField(StringBuilder out, String name, String value) {
+    out.append('"').append(name).append("\":\"");
+    appendEscaped(out, value);
+    out.append('"');
   }
 
-  private static String escape(String s) {
-    StringBuilder b = new StringBuilder(s.length() + 8);
-    for (int i = 0; i < s.length(); i++) {
-      char c = s.charAt(i);
+  private static void appendIntField(StringBuilder out, String name, int value) {
+    out.append('"').append(name).append("\":").append(value);
+  }
+
+  /**
+   * Appends {@code value} to {@code out} with JSON-string escaping applied. Handles the six
+   * mandatory escape sequences and emits {@code &#92;uXXXX} for control characters below {@link
+   * #FIRST_PRINTABLE_ASCII}.
+   */
+  private static void appendEscaped(StringBuilder out, String value) {
+    for (int i = 0; i < value.length(); i++) {
+      char c = value.charAt(i);
       switch (c) {
-        case '\\' -> b.append("\\\\");
-        case '"' -> b.append("\\\"");
-        case '\n' -> b.append("\\n");
-        case '\r' -> b.append("\\r");
-        case '\t' -> b.append("\\t");
-        default -> {
-          if (c < 0x20) b.append(String.format("\\u%04x", (int) c));
-          else b.append(c);
-        }
+        case '\\' -> out.append("\\\\");
+        case '"' -> out.append("\\\"");
+        case '\n' -> out.append("\\n");
+        case '\r' -> out.append("\\r");
+        case '\t' -> out.append("\\t");
+        default -> appendUnicodeOrLiteral(out, c);
       }
     }
-    return b.toString();
+  }
+
+  private static void appendUnicodeOrLiteral(StringBuilder out, char c) {
+    if (c < FIRST_PRINTABLE_ASCII) {
+      out.append(String.format("\\u%04x", (int) c));
+    } else {
+      out.append(c);
+    }
   }
 }