fix(chat): surface silent generations instead of empty bubbles

Qwen3:14b sometimes burns output tokens on tool-calling attempts whose
emission doesn't parse into any field we read — eval_count > 0 but no
thinking/content/tool_calls ever stream to the caller. Generation
completes "successfully," the user sees an empty assistant bubble, and
no error is logged. Seen in conv 220 today.

Two safety rails:

- stream_chat_with_tools now tracks whether it yielded anything; when
  Ollama's done frame reports eval_count > 0 with zero yields, log a
  warning including the last ~5 raw frames so the next occurrence leaves
  breadcrumbs for diagnosis.

- run_generation checks the same post-condition after the tool loop
  exits and, if content is empty with no tool calls but output_tokens
  > 0, substitutes a visible fallback message and streams it as a chunk
  so the user gets something readable instead of a blank bubble.

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

src/fabledassistant/services/generation_task.py

@@ -542,6 +542,29 @@ async def run_generation(
         # Strip model artifacts from final content
         buf.content_so_far = _TOOL_CALL_MARKER.sub("", buf.content_so_far)
 
+        # Silent-generation safety net: the model burned output tokens but
+        # nothing landed in content or tool_calls (seen with qwen3:14b when
+        # its tool-call emission doesn't parse). Show a visible fallback so
+        # the user isn't staring at an empty bubble.
+        if (
+            not cancelled
+            and not buf.content_so_far.strip()
+            and not all_tool_calls
+            and (timing.get("output_tokens") or 0) > 0
+        ):
+            logger.warning(
+                "Silent generation for conv %d: output_tokens=%s but empty content "
+                "and no tool calls (model=%s)",
+                conv_id, timing.get("output_tokens"), model,
+            )
+            fallback = (
+                "I wasn't able to produce a usable response — the model generated "
+                "tokens that couldn't be parsed as content or a tool call. "
+                "Please try rephrasing, or try again."
+            )
+            buf.content_so_far = fallback
+            buf.append_event("chunk", {"chunk": fallback})
+
         # Final save
         logger.info("Generation complete for conv %d: content_length=%d, tool_calls=%d",
                      conv_id, len(buf.content_so_far), len(all_tool_calls))