feat: enhance callModel to accept chat-style messages

[mattapperson]

Summary

• Add support for chat-style Message[] input in addition to responses-style input
• Add support for chat-style ToolDefinitionJson[] tools (OpenAI format)
• Automatically convert chat-style messages and tools to responses format when detected
• Handle all message types: system, user, assistant, developer, and tool
• Export CallModelInput and CallModelTools types for users

Usage

import { Message } from "@openrouter/ai-sdk-provider";

// Chat-style messages and tools (OpenAI format)
const response = client.callModel({
  model: "openai/gpt-4",
  input: [
    { role: "system", content: "You are helpful." },
    { role: "user", content: "Get weather in Paris" },
  ] as Message[],
  tools: [
    {
      type: "function",
      function: {
        name: "get_weather",
        description: "Get weather for a location",
        parameters: {
          type: "object",
          properties: {
            location: { type: "string" }
          },
          required: ["location"]
        }
      }
    }
  ]
});

// Still supports responses-style input and tools
const response2 = client.callModel({
  model: "openai/gpt-4",
  input: "Hello!",
});

Test plan

• TypeScript compiles without errors
• Added tests for chat-style message support
• Added tests for chat-style tools support
• Added tests for combined chat-style messages and tools