Azure TTS _handle_canceled was putting None (the normal completion
signal) into the audio queue for all cancellation reasons, so run_tts
treated errors identically to success—silently producing no audio.
Now error cancellations put an Exception marker in the queue, which
run_tts converts to an ErrorFrame.
Azure STT had no canceled event handler at all, so auth failures,
network errors, and rate-limit cancellations were invisible. Added
_on_handle_canceled which pushes an ErrorFrame upstream via push_error.
Fixespipecat-ai/pipecat#3892
