Block preemptive generation during tool execution

[Genmin]

Summary

Prevents preemptive generation from starting while a previous speech handle is still executing function tools in the background.

_currentSpeech is cleared after the assistant generation/playout phase, before tool execution finishes. During that window a user EOU could previously start a preemptive reply from stale chat context, before the tool result had been inserted. The existing _backgroundSpeeches set already tracks that exact interval, so the preemptive guard now treats it as active work.

Fixes #1365.

Validation

• pnpm vitest run 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 exec eslint -f unix agents/src/voice/agent_activity.ts agents/src/voice/agent_activity.test.ts (existing warnings only)
• pnpm --filter @livekit/agents build
• git diff --check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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