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/abort.test.ts

@@ -25,7 +25,7 @@ async function testAbortSignal<TApi extends Api>(llm: Model<TApi>, options: Opti
 	const msg = await response.result();
 
 	// If we get here without throwing, the abort didn't work
-	expect(msg.stopReason).toBe("error");
+	expect(msg.stopReason).toBe("aborted");
 	expect(msg.content.length).toBeGreaterThan(0);
 
 	context.messages.push(msg);
@@ -46,7 +46,7 @@ async function testImmediateAbort<TApi extends Api>(llm: Model<TApi>, options: O
 	};
 
 	const response = await complete(llm, context, { ...options, signal: controller.signal });
-	expect(response.stopReason).toBe("error");
+	expect(response.stopReason).toBe("aborted");
 }
 
 describe("AI Providers Abort Tests", () => {