feat: add strongly typed tools with createTool helpers

[mattapperson]

Summary

• Add tool factory function for creating typed tool definitions
• Add type inference helpers: InferToolInput, InferToolOutput, InferToolEvent, InferToolEventsUnion
• Add TypedToolCall and TypedToolCallUnion for typed tool call results
• Make ResponseWrapper generic with TTools parameter to preserve type information
• Make callModel generic to flow tool types through the call chain
• Make event types generic (ToolStreamEvent<TEvent>, ChatStreamEvent<TEvent>, etc.) for typed preliminary results
• Export all new types from index.ts
• Add example demonstrating typed tool calling with generator events

How It Works

When using as const with the tools array, all type information flows through:

const weatherTool = tool({
  name: "get_weather",
  inputSchema: z.object({ location: z.string() }),
  outputSchema: z.object({ temperature: z.number() }),
  execute: async (params) => {
    // params is typed as { location: string }
    return { temperature: 72 };
  },
});

const result = client.callModel({
  model: "openai/gpt-4",
  input: "What's the weather?",
  tools: [weatherTool] as const,
});

// Tool calls are typed!
const toolCalls = await result.getToolCalls();
// toolCalls[0].arguments is typed as { location: string }

// Generator events are typed!
for await (const event of result.getToolStream()) {
  if (event.type === "preliminary_result") {
    // event.result is typed based on eventSchema
  }
}

Test plan

• Build passes (npm run build)
• Lint passes (npm run lint)
• Unit tests pass (npx vitest run tests/unit)
• Type inference works correctly with as const