fix(core): make default user span time explicit

[chenghao-mou]

tracer.start_span has a default value when start_time is None, so we need a default value for _user_turn_start as well to make sure they are consistent.

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

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[toubatbrian]

This is an automated Claude Code Routine created by @toubatbrian. Right now it is in experimentation stage. The automation reviewed this PR for an auto-port into agents-js.

Decision: skipping the port.

The Python fix targets a consistency issue specific to the Python implementation: previously, calling _ensure_user_turn_span(start_time=None) would let tracer.start_span(..., start_time=None) fall back to the OTel SDK default while self._user_turn_start stayed None, leaving the span's actual start time and the separately-tracked _user_turn_start field out of sync (the latter is what _bounce_eou_task consumes for EOU metrics). The fix materializes start_time = time.time() once and assigns both from it.

The equivalent code in agents-js (agents/src/voice/audio_recognition.ts, ensureUserTurnSpan) does not have a sibling userTurnStart field that needs to stay in sync with the span's start time:

• The bounceEOUTask EOU metric path uses this.speechStartTime — which is populated exclusively by the VAD INFERENCE_DONE handler, backdated by ev.rawAccumulatedSpeech, and reset only on commit / clear. Its lifecycle is independent of ensureUserTurnSpan.
• ensureUserTurnSpan(startTime?) forwards startTime directly to tracer.startSpan({ ..., startTime }). When undefined, the OTel JS SDK already substitutes hrtime() internally, matching the Python post-fix behavior at the span level.

So in agents-js the two-field inconsistency the Python PR fixes does not exist; mirroring the diff verbatim would be a no-op (an explicit Date.now() fallback that the SDK already does). No port action needed.

— Automated routine

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