Use typeguard for FrameProcessor check

[lukasIO]

Description

this avoids unreliable instanceof check and replaces it with a typeguard. Depends on livekit/node-sdks#611

Changes Made

Pre-Review Checklist

• Build passes: All builds (lint, typecheck, tests) pass locally
• AI-generated code reviewed: Removed unnecessary comments and ensured code quality
• Changes explained: All changes are properly documented and justified above
• Scope appropriate: All changes relate to the PR title, or explanations provided for why they're included
• Video demo: A small video demo showing changes works as expected and did not break any existing functionality using Agent Playground (if applicable)

Testing

• Automated tests added/updated (if applicable)
• All tests pass
• Make sure both restaurant_agent.ts and realtime_agent.ts work properly (for major changes)

Additional Notes

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Note to reviewers: Please ensure the pre-review checklist is completed before starting your review.

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Latest commit: 28b710a

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