Tracking Issue: Add close() cleanup to STT/TTS plugins

[KrishnaShuk]

Describe the bug

Following the pattern from PR #874, several STT and TTS plugins are missing a close() implementation. This can cause hanging network requests or resource leaks when a session ends.

I plan to add the AbortController cancellation pattern to the remaining plugins.

Cleanup Checklist:

TTS Plugins:

• openai.TTS : feat(openai): implement close() for openai TTS #883
• elevenlabs.TTS
• google.TTS : feat(gemini & rime): implemented close() for google and rime tts #890
• cartesia.TTS
• rime.TTS : feat(gemini & rime): implemented close() for google and rime tts #890
• neuphonic.TTS
• deepgram.TTS

STT Plugins:

• openai.STT : feat(openai): implement close() cleanup for STT  #874

Relevant log output

No response

Describe your environment

• @livekit/agents version: latest
• Node.js version: 20+

Minimal reproducible example

No response

Additional information

No response