refactor: simplify and harden PR fixes after review

- Fix cron ThreadPoolExecutor blocking on timeout: use shutdown(wait=False,
  cancel_futures=True) instead of context manager that waits indefinitely
- Extract _dequeue_pending_text() to deduplicate media-placeholder logic
  in interrupt and normal-completion dequeue paths
- Remove hasattr guards for _running_agents_ts: add class-level default
  so partial test construction works without scattered defensive checks
- Move `import concurrent.futures` to top of cron/scheduler.py
- Progress throttle: sleep remaining interval instead of busy-looping
  0.1s (~15 wakeups per 1.5s window → 1 wakeup)
- Deduplicate _load_stt_config() in transcription_tools.py:
  _has_openai_audio_backend() now delegates to _resolve_openai_audio_client_config()

tools/transcription_tools.py

@@ -128,10 +128,11 @@ def is_stt_enabled(stt_config: Optional[dict] = None) -> bool:
 
 def _has_openai_audio_backend() -> bool:
     """Return True when OpenAI audio can use config credentials, env credentials, or the managed gateway."""
-    stt_config = _load_stt_config()
-    openai_cfg = stt_config.get("openai", {})
-    cfg_api_key = openai_cfg.get("api_key", "")
-    return bool(cfg_api_key or resolve_openai_audio_api_key() or resolve_managed_tool_gateway("openai-audio"))
+    try:
+        _resolve_openai_audio_client_config()
+        return True
+    except ValueError:
+        return False
 
 
 def _find_binary(binary_name: str) -> Optional[str]: