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.

open-sse/providers/registry/perplexity-agent.js

@@ -0,0 +1,49 @@
+export default {
+  id: "perplexity-agent",
+  priority: 181,
+  alias: "perplexity-agent",
+  aliases: [
+    "pplx-agent",
+    "pplx-responses",
+  ],
+  uiAlias: "pa",
+  display: {
+    name: "Perplexity Agent",
+    icon: "travel_explore",
+    color: "#20808D",
+    textIcon: "PA",
+    website: "https://www.perplexity.ai",
+    notice: {
+      text: "Perplexity Agent API exposes GPT, Claude, Gemini, Grok, GLM, Kimi, and Sonar models through one OpenAI-compatible Responses API.",
+      apiKeyUrl: "https://www.perplexity.ai/settings/api",
+    },
+  },
+  category: "apikey",
+  authType: "apikey",
+  transport: {
+    baseUrl: "https://api.perplexity.ai/v1/responses",
+    validateUrl: "https://api.perplexity.ai/v1/models",
+    format: "openai-responses",
+  },
+  models: [
+    { id: "perplexity/sonar", name: "Perplexity Sonar" },
+    { id: "openai/gpt-5.5", name: "GPT-5.5" },
+    { id: "openai/gpt-5.4", name: "GPT-5.4" },
+    { id: "openai/gpt-5.4-mini", name: "GPT-5.4 Mini" },
+    { id: "anthropic/claude-sonnet-4-6", name: "Claude Sonnet 4.6" },
+    { id: "anthropic/claude-opus-4-8", name: "Claude Opus 4.8" },
+    { id: "google/gemini-3.1-pro-preview", name: "Gemini 3.1 Pro" },
+    { id: "xai/grok-4.20-reasoning", name: "Grok 4.20 Reasoning" },
+    { id: "perplexity/glm-5.2", name: "GLM 5.2" },
+    { id: "perplexity/kimi-k2.7-code", name: "Kimi K2.7 Code" },
+    { id: "nvidia/nemotron-3-super-120b-a12b", name: "Nemotron 3 Super 120B" },
+  ],
+  serviceKinds: ["llm", "webSearch"],
+  searchViaChat: {
+    defaultModel: "perplexity/sonar",
+    endpoint: "https://api.perplexity.ai/v1/responses",
+    pricingUrl: "https://docs.perplexity.ai/docs/agent-api/models",
+  },
+  modelsFetcher: { url: "https://api.perplexity.ai/v1/models", type: "openai" },
+  passthroughModels: true,
+};