refactor(ai): improve error handling and stop reason types

- Add 'aborted' as a distinct stop reason separate from 'error'
- Change AssistantMessage.error to errorMessage for clarity
- Update error event to include reason field ('error' | 'aborted')
- Map provider-specific safety/refusal reasons to 'error' stop reason
- Reorganize utility functions into utils/ directory
- Rename agent.ts to agent-loop.ts for better clarity
- Fix error handling in all providers to properly distinguish abort from error

packages/ai/test/enum-test.ts

@@ -1,7 +1,7 @@
 import { Type } from "@sinclair/typebox";
 import { z } from "zod";
 import { zodToJsonSchema } from "zod-to-json-schema";
-import { StringEnum } from "../src/typebox-helpers.js";
+import { StringEnum } from "../src/utils/typebox-helpers.js";
 
 // Zod version
 const zodSchema = z.object({