feat(core): add plain text chat content

[rosetta-livekit-bot]

Summary

• Port Python ChatMessage.plain_text_content as JS ChatMessage.plainTextContent
• Strip expressive markup from assistant turns when summarizing chat history and preparing EOU context
• Add stripMarkup serialization support for remote multilingual EOU requests

Port of livekit/agents#6348.

Verification

• pnpm prettier --check "agents/src/llm/chat_context.ts" "plugins/livekit/src/turn_detector/base.ts" "plugins/livekit/src/turn_detector/multilingual.ts" ".changeset/plain-chat-content.md"
• pnpm --filter @livekit/agents typecheck
• pnpm --filter @livekit/agents build
• pnpm --filter @livekit/agents-plugin-livekit build:types
• pnpm test -- agents/src/llm/chat_context.test.ts
• pnpm --filter @livekit/agents lint (warning-only existing lint output)
• pnpm --filter @livekit/agents-plugin-livekit lint (warning-only existing lint output)
• cue-cli text-mode smoke: drove a temporary JS agent and resolved conversation_item_added(.item.message.role="ASSISTANT")

---

Ported from livekit/agents#6348

Original PR description

No description.

[changeset-bot]

🦋 Changeset detected

Latest commit: cf07bd1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 36 packages

Name	Type
@livekit/agents	Major
@livekit/agents-plugin-livekit	Major
@livekit/agents-plugin-anam	Major
@livekit/agents-plugin-anthropic	Major
@livekit/agents-plugin-assemblyai	Major
@livekit/agents-plugin-baseten	Major
@livekit/agents-plugin-bey	Major
@livekit/agents-plugin-cartesia	Major
@livekit/agents-plugin-cerebras	Major
@livekit/agents-plugin-deepgram	Major
@livekit/agents-plugin-did	Major
@livekit/agents-plugin-elevenlabs	Major
@livekit/agents-plugin-fishaudio	Major
@livekit/agents-plugin-google	Major
@livekit/agents-plugin-hedra	Major
@livekit/agents-plugin-hume	Major
@livekit/agents-plugin-inworld	Major
@livekit/agents-plugin-lemonslice	Major
@livekit/agents-plugin-liveavatar	Major
@livekit/agents-plugin-minimax	Major
@livekit/agents-plugin-mistral	Major
@livekit/agents-plugin-mistralai	Major
@livekit/agents-plugin-neuphonic	Major
@livekit/agents-plugin-openai	Major
@livekit/agents-plugin-perplexity	Major
@livekit/agents-plugin-phonic	Major
@livekit/agents-plugin-resemble	Major
@livekit/agents-plugin-rime	Major
@livekit/agents-plugin-runway	Major
@livekit/agents-plugin-sarvam	Major
@livekit/agents-plugin-silero	Major
@livekit/agents-plugin-soniox	Major
@livekit/agents-plugin-tavus	Major
@livekit/agents-plugins-test	Major
@livekit/agents-plugin-trugen	Major
@livekit/agents-plugin-xai	Major

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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.