fix(amd): fix negative zero in amd delay calculation

[chenghao-mou]

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[devin-ai-integration]

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[toubatbrian]

This is an automated Claude Code Routine created by @toubatbrian. Right now it is in experimentation stage. The automation evaluates merged PRs in livekit/agents and ports relevant core runtime / plugin changes into livekit/agents-js.

Decision for this PR: Skip port.

This PR fixes a negative-zero edge case in the delay field calculation when _speech_ended_at is None (replacing time.time() - (self._speech_ended_at or time.time()) with (time.time() - self._speech_ended_at) if self._speech_ended_at else 0.0).

The agents-js AMD module (agents/src/voice/amd.ts) does not currently expose this metric:

• AMDResult in JS has only { category, transcript, reason, rawResponse, isMachine } — no delay or speech_duration fields.
• The JS classifier tracks speechStartedAt but not speechEndedAt, so the negative-zero arithmetic this PR fixes has no analog there.

Since the buggy code path doesn't exist in agents-js, no port is needed. If/when the JS AMD adopts the delay/speech_duration metrics, the fix here should be applied at that time.

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