fix(ChatMessage): restore IllegalArgumentException on null parts list

Commit f97c85d (spotbugs RCN cleanup) removed the null branch inside
requireNonEmpty under the assumption that the JSpecify @nonnull default
made parts==null unreachable. The deliberately type-cast call site in
MultimodalMessagesTest.nullPartsListIsRejected bypasses that static
check at compile time, so the runtime path now NPEs inside
requireNonEmpty(parts).isEmpty() instead of throwing the documented
IllegalArgumentException.

Restore the contract by validating null at the public-constructor
boundary (the right place for external-input validation) and keep the
private requireNonEmpty focused on the empty case.

Author: claude

src/main/java/net/ladenthin/llama/ChatMessage.java

@@ -77,12 +77,19 @@ public ChatMessage(String role, String content, @Nullable String toolCallId, Lis
     public ChatMessage(String role, List<ContentPart> parts) {
         this(
                 role,
-                concatText(parts),
+                concatText(requireNonNull(parts)),
                 null,
                 Collections.<ToolCall>emptyList(),
                 Collections.unmodifiableList(new java.util.ArrayList<ContentPart>(requireNonEmpty(parts))));
     }
 
+    private static List<ContentPart> requireNonNull(List<ContentPart> parts) {
+        if (parts == null) {
+            throw new IllegalArgumentException("parts must not be null");
+        }
+        return parts;
+    }
+
     private ChatMessage(
             String role,
             String content,