fix(chat): omit attachments key from createMessage when no files provided

Object.keys({ attachments: undefined }) still includes "attachments" as a
key, breaking the "returns a plain object with expected keys" test. Fix by
conditionally spreading attachments only when non-empty, and Object.freeze
the return value to preserve the existing immutability assertion.

Fixes 2 test cases in createMessage.test.ts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/tabs/chat/types.ts

@@ -26,13 +26,16 @@ export function createMessage(
   content: string,
   attachments?: ChatAttachment[],
 ): ChatMessage {
-  return {
+  const base = {
     id: crypto.randomUUID(),
     role,
     content,
-    attachments: attachments && attachments.length > 0 ? attachments : undefined,
     timestamp: new Date().toISOString(),
   };
+  if (attachments && attachments.length > 0) {
+    return Object.freeze({ ...base, attachments });
+  }
+  return Object.freeze(base);
 }
 
 // appendMessageDeduped adds a ChatMessage to `prev` unless the tail