feat(perplexity): add Agent API provider (#2492)

Add perplexity-agent provider using OpenAI-compatible Responses API,
routing third-party models (GPT, Claude, Gemini, Grok, GLM, Kimi, Sonar)
through one endpoint. Expose /v1/models discovery, add chat-search wrapper
via web_search tool. Existing Sonar provider unchanged.

src/app/api/providers/[id]/models/route.js

@@ -236,6 +236,7 @@ const PROVIDER_MODELS_CONFIG = {
   xai: createOpenAIModelsConfig("https://api.x.ai/v1/models"),
   mistral: createOpenAIModelsConfig("https://api.mistral.ai/v1/models"),
   perplexity: createOpenAIModelsConfig("https://api.perplexity.ai/v1/models"),
+  "perplexity-agent": createOpenAIModelsConfig("https://api.perplexity.ai/v1/models"),
   together: createOpenAIModelsConfig("https://api.together.xyz/v1/models"),
   fireworks: createOpenAIModelsConfig("https://api.fireworks.ai/inference/v1/models"),
   cerebras: createOpenAIModelsConfig("https://api.cerebras.ai/v1/models"),