fix(voice): honor agent preemptive generation options

[rosetta-livekit-bot]

Summary

• merge agent-level preemptiveGeneration options over resolved session options in AgentActivity
• use the merged options for preemptive LLM generation and preemptive TTS
• cover the upstream agent/session override cases in the existing preemptive-generation tests

Testing

• pnpm test agents/src/voice/agent_activity.test.ts
• pnpm exec prettier --check agents/src/voice/agent_activity.ts agents/src/voice/agent_activity.test.ts
• pnpm build:agents

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Ported from livekit/agents#6114

Original PR description

Closes #6112.

preemptive_generation was read directly from session.options, so an agent's turn_handling={"preemptive_generation": {...}} had no effect — the session value (default enabled: True) always won. This is inconsistent with endpointing, which already resolves agent-over-session via endpointing_opts.

This adds a preemptive_generation_opts property on AgentActivity that merges the agent's keys over the fully-resolved session value, and routes both call sites (on_preemptive_generation and the preemptive-TTS branch in pipeline_reply) through it.

Note this only fixes the effective behavior. The session report (make_session_report) still reflects session-level options only, so it won't show per-agent overrides; that reporting gap is tracked separately.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 06da8b3

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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