deepgram stt PREFLIGHT_TRANSCRIPT event

[simllll]

rebase after #783

Description

checks deepgram eager to return event for preemptive generation

Changes Made

took over the python implementatoin https://github.com/livekit/agents/blob/d19519d602eaae8f212d4f93252c05792b7b3a66/livekit-plugins/livekit-plugins-deepgram/livekit/plugins/deepgram/stt_v2.py#L469

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

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: b1f3b69

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