fix(frontend): drop dead identity ternary and add opt-in export tests

Address review feedback on the previous export commit:

1. Removed the no-op `typeof msg.content === "string" ? msg.content : msg.content`
   expression in `formatThreadAsJSON`. Both branches returned the same value;
   the message content now flows through unchanged whether it is a string or
   the rich `MessageContent[]` shape (LangChain JSON-serialises the array
   structure correctly already).

2. Expanded the JSDoc on `ExportOptions` to make it clearer that the four
   flags are not currently wired to any UI control — callers wanting a debug
   export must build the options object explicitly. The default behaviour
   continues to match the explicit prescription in
   bytedance/deer-flow#3107 BUG-006.

3. Added opt-in coverage. The previous tests only exercised the
   `options = {}` default path; the new cases verify each flag flips the
   corresponding payload back into the export so a future debug-export
   surface does not silently break the contract.

Refs: bytedance/deer-flow#3107 (BUG-006)

frontend/src/core/threads/export.ts

@@ -15,12 +15,13 @@ import { titleOfThread } from "./utils";
 /**
  * Optional debug switches for advanced exports.
  *
- * Bytedance/deer-flow issue #3107 BUG-006: by default, the user-facing chat
- * export must include only the visible transcript. Internal payloads —
- * `hide_from_ui` messages, reasoning content, tool calls, and tool result
- * messages — stay out unless the caller explicitly opts in. There is no UI
- * surface for this today; the flags exist so a future "debug export" can
- * reuse the same formatter instead of forking it.
+ * Bytedance/deer-flow issue #3107 BUG-006 explicitly prescribes that the
+ * default export includes only the user-visible transcript and excludes
+ * thinking/reasoning content, tool calls, tool results, hidden messages,
+ * memory injection, and `<system-reminder>` payloads. These options let a
+ * future "debug export" surface re-include any of those categories without
+ * forking the formatter. They are not currently wired to any UI control —
+ * callers that want them must construct the options object explicitly.
  */
 export interface ExportOptions {
   includeReasoning?: boolean;
@@ -134,7 +135,7 @@ export function formatThreadAsJSON(
     messages: visibleMessages(messages, options).map((msg) => ({
       type: msg.type,
       id: msg.id,
-      content: typeof msg.content === "string" ? msg.content : msg.content,
+      content: msg.content,
       ...(options.includeToolCalls &&
       msg.type === "ai" &&
       msg.tool_calls?.length