refactor: Address SonarQube findings on body parsing helpers

- ContentTypeHeader.parameter: replace dual-continue loop with a single
  early-return guarded by an inline key check; extract quote-stripping
  into a small unquote() helper.
- FormUrlEncodedParser.decodeComponent: chain the original
  IllegalArgumentException via initCause() on the rethrown
  ValidationException so the underlying cause is preserved.
- FormUrlEncodedParser.addEntry: replace get()+null-check+put() with
  Map.merge(), which is the right primitive for collision handling.
- TextPlainParserTest: use isEmpty() instead of isEqualTo("").

Author: thced

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

@@ -31,23 +31,21 @@ public static Optional<String> parameter(String header, String name) {
     if (semi < 0) {
       return Optional.empty();
     }
-    String[] parts = header.substring(semi + 1).split(";");
-    for (String p : parts) {
+    for (String p : header.substring(semi + 1).split(";")) {
       String trimmed = p.trim();
       int eq = trimmed.indexOf('=');
-      if (eq <= 0) {
-        continue;
+      String key = (eq <= 0) ? "" : trimmed.substring(0, eq).trim().toLowerCase(Locale.ROOT);
+      if (key.equals(target)) {
+        return Optional.of(unquote(trimmed.substring(eq + 1).trim()));
       }
-      String key = trimmed.substring(0, eq).trim().toLowerCase(Locale.ROOT);
-      if (!key.equals(target)) {
-        continue;
-      }
-      String value = trimmed.substring(eq + 1).trim();
-      if (value.length() >= 2 && value.startsWith("\"") && value.endsWith("\"")) {
-        value = value.substring(1, value.length() - 1);
-      }
-      return Optional.of(value);
     }
     return Optional.empty();
   }
+
+  private static String unquote(String value) {
+    if (value.length() >= 2 && value.startsWith("\"") && value.endsWith("\"")) {
+      return value.substring(1, value.length() - 1);
+    }
+    return value;
+  }
 }

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

@@ -44,28 +44,31 @@ private static String decodeComponent(String value, Charset charset) {
     try {
       return URLDecoder.decode(value, charset);
     } catch (IllegalArgumentException ex) {
-      throw new ValidationException(
-          new ValidationError(
-              "/body", "decode", "malformed form URL encoding: " + ex.getMessage(), value));
+      ValidationException ve =
+          new ValidationException(
+              new ValidationError(
+                  "/body", "decode", "malformed form URL encoding: " + ex.getMessage(), value));
+      ve.initCause(ex);
+      throw ve;
     }
   }
 
   private static void addEntry(Map<String, Object> out, String key, String value) {
-    Object existing = out.get(key);
-    if (existing == null) {
-      out.put(key, value);
-      return;
-    }
-    if (existing instanceof List<?> list) {
-      @SuppressWarnings("unchecked")
-      List<String> typed = (List<String>) list;
-      typed.add(value);
-      return;
-    }
-    List<String> list = new ArrayList<>();
-    list.add((String) existing);
-    list.add(value);
-    out.put(key, list);
+    out.merge(
+        key,
+        value,
+        (existing, incoming) -> {
+          if (existing instanceof List<?> list) {
+            @SuppressWarnings("unchecked")
+            List<String> typed = (List<String>) list;
+            typed.add((String) incoming);
+            return typed;
+          }
+          List<String> merged = new ArrayList<>();
+          merged.add((String) existing);
+          merged.add((String) incoming);
+          return merged;
+        });
   }
 
   /** Returns the parsed map after coercing field values against the given body schema. */

src/test/java/com/retailsvc/http/internal/TextPlainParserTest.java

@@ -24,7 +24,7 @@ void respectsCharsetFromHeader() {
 
   @Test
   void emptyBodyDecodesToEmptyString() {
-    assertThat(parser.parse(new byte[0], null)).isEqualTo("");
+    assertThat(parser.parse(new byte[0], null)).isEmpty();
   }
 
   @Test