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/src/agent/index.ts

@@ -1,3 +1,3 @@
-export { type AgentEvent, type PromptConfig, prompt } from "./agent";
+export { prompt } from "./agent";
 export * from "./tools";
-export type { AgentContext, AgentTool } from "./types";
+export type { AgentContext, AgentEvent, AgentTool, PromptConfig } from "./types";