Add Perplexity Agent API provider

[kesku]

Add new perplexity-agent provider using @ai-sdk/openai with the Perplexity Agent API (/v1/responses)

• 15 models across 5 vendors:
  • Anthropic: claude-opus-4-6, claude-opus-4-5, claude-sonnet-4-6, claude-sonnet-4-5, claude-haiku-4-5
  • OpenAI: gpt-5.2, gpt-5.1, gpt-5-mini
  • Google: gemini-3.1-pro-preview, gemini-3-pro-preview, gemini-3-flash-preview, gemini-2.5-pro, gemini-2.5-flash
  • xAI: grok-4-1-fast-non-reasoning
  • Perplexity: sonar
• All models support tool calling via the Agent API
• Reuses existing Perplexity logo

This is separate from the existing perplexity provider which uses @ai-sdk/perplexity for the Sonar chat completions endpoint (/chat/completions). The Agent API is OpenAI Responses API-compatible and requires @ai-sdk/openai.

You can run

curl -s https://api.perplexity.ai/v1/models -H "Authorization: Bearer $PERPLEXITY_API_KEY" | jq

to verify models