(sarvam stt): fix end of speech timing

[tinalenguyen]

Problem

The Sarvam streaming STT plugin tried to manufacture an audio-relative speech-end time from two sources that don't actually provide one:

• The VAD END_SPEECH event's occured_at field — which logging proved is a wall-clock Unix epoch, not an audio-stream offset, so it was rejected 100% of the time by a range-check heuristic.
• A local send-clock counter (_audio_position) that counts audio uploaded, not processed — biased and fabricated.

Sarvam's streaming socket genuinely sends no usable word timing (no timestamps array; speech_start/speech_end come back null), so all this machinery produced misleading timestamps.

Change

Aligned Sarvam with how every other STT plugin (Deepgram, AssemblyAI, Google, Azure…) handles this:

1. END_OF_SPEECH is now emitted bare — no alternatives/end_time. Previously it carried an empty-text SpeechData with a fabricated end_time + unused speech_end_wall_time metadata.
2. FINAL_TRANSCRIPT timing comes only from the provider — start_time/end_time read from speech_start/speech_end, falling back to 0.0. The voice pipeline then uses wall-clock for EOU timing (standard behavior for providers without streaming word timing).
3. Deleted the dead machinery: _interpret_signal_time, _resolved_speech_end, the _audio_position send-clock (+ its hot-loop increment), _utterance_server_speech_end, _utterance_speech_end_audio_pos, _utterance_speech_end_wall, and the require_end_time param.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[chenghao-mou]

lgtm, one small nitpick.

[chenghao-mou]

we can add the start_time_offset in _positive_time so that all values through that function includes session start -> audio start delay:

    def _positive_time(self, value: object) -> float | None:
        if isinstance(value, bool) or not isinstance(value, (int, float)):
            return None
        if value <= 0:
            return None
        return float(value) + self.start_time_offset