feat(ai): Implement Zod-based tool validation and improve Agent API

- Replace JSON Schema with Zod schemas for tool parameter definitions
- Add runtime validation for all tool calls at provider level
- Create shared validation module with detailed error formatting
- Update Agent API with comprehensive event system
- Add agent tests with calculator tool for multi-turn execution
- Add abort test to verify proper handling of aborted requests
- Update documentation with detailed event flow examples
- Rename generate.ts to stream.ts for clarity

packages/ai/test/empty.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
-import { complete } from "../src/generate.js";
 import { getModel } from "../src/models.js";
+import { complete } from "../src/stream.js";
 import type { Api, AssistantMessage, Context, Model, OptionsForApi, UserMessage } from "../src/types.js";
 
 async function testEmptyMessage<TApi extends Api>(llm: Model<TApi>, options: OptionsForApi<TApi> = {}) {