Fix streaming for z-ai in anthropic provider, add preliminary support for tool call streaming. Only reporting argument string deltas, not partial JSON objects

packages/ai/src/agent/index.ts

@@ -0,0 +1,3 @@
+export { type AgentEvent, type PromptConfig, prompt } from "./agent";
+export * from "./tools";
+export type { AgentContext, AgentTool } from "./types";