docs(google): clarify Gemini 3.1 realtime limitations

[rahulmanuwas]

Summary

• document that generateReply() is not currently compatible with gemini-3.1-flash-live-preview
• explain that this impacts voice.AgentSession flows that rely on programmatic reply generation
• clarify that thinkingConfig semantics are model-specific: Gemini 3.1 uses thinkingLevel, while Gemini 2.5 uses thinkingBudget
• improve the runtime error message to point users toward a Gemini 2.5 live model when they need generateReply()-driven flows

Closes #1197.

Validation

• pnpm --filter @livekit/agents-plugin-google... build
• pnpm vitest run plugins/google/src/llm.test.ts plugins/google/src/beta/gemini_tts.test.ts (the Gemini TTS test still requires GOOGLE_API_KEY in this environment)

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