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/handoff.test.ts

@@ -1,19 +1,16 @@
 import { describe, expect, it } from "vitest";
-import { complete } from "../src/generate.js";
+import { z } from "zod";
 import { getModel } from "../src/models.js";
+import { complete } from "../src/stream.js";
 import type { Api, AssistantMessage, Context, Message, Model, Tool, ToolResultMessage } from "../src/types.js";
 
 // Tool for testing
 const weatherTool: Tool = {
 	name: "get_weather",
 	description: "Get the weather for a location",
-	parameters: {
-		type: "object",
-		properties: {
-			location: { type: "string", description: "City name" },
-		},
-		required: ["location"],
-	},
+	parameters: z.object({
+		location: z.string().describe("City name"),
+	}),
 };
 
 // Pre-built contexts representing typical outputs from each provider