fix(research): show exception type when error message is empty

Some exceptions (e.g. connection errors) produce empty str(e),
resulting in "Research failed: " with no explanation. Fall back to
the exception class name when the message is blank.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/generation_task.py

@@ -376,8 +376,9 @@ async def run_generation(
                                 buf.content_so_far += done_text
                             except Exception as e:
                                 logger.exception("Research pipeline failed for topic: %s", topic)
-                                result = {"success": False, "error": str(e)}
-                                err_text = f"\nResearch failed: {e}"
+                                err_msg = str(e) or f"{type(e).__name__}: unexpected error"
+                                result = {"success": False, "error": err_msg}
+                                err_text = f"\nResearch failed: {err_msg}"
                                 buf.append_event("chunk", {"chunk": err_text})
                                 buf.content_so_far += err_text
                             research_completed = True