Add first interim STT latency metric

[spyrux]

closes #6533 — STT time-to-first-partial metric (#6533)

Summary

Adds first_interim_delay: the elapsed time from VAD detecting speech onset to the first interim (partial) STT
transcript for a user turn.

The metric is:

• captured once, on the first interim STT event for each turn
• reset when the user turn is cleared or completed
• exposed in EOUMetrics
• included in the per-turn metrics report
• recorded as the lk.agents.turn.first_interim_delay OpenTelemetry histogram

A missing interim transcript is represented as unavailable in the turn report and 0.0 in the existing EOUMetrics
event convention.

Tests

pytest tests/test_end_of_turn_metrics.py tests/test_stt_first_interim_metric.py

- verifies the delay is calculated from speech onset
- verifies the metric is unavailable when no interim transcript arrives

[CLAassistant]

All committers have signed the CLA.

[Sanjays2402]

This marks an interim as received before checking that it contains any text. Deepgram v1 forwards non-final Results even when alt["transcript"] is empty, so the first blank result wins the timestamp and the metric no longer measures first transcript latency; gate this on ev.alternatives[0].text.