feat(ai): Add start event emission to all providers

- Emit start event with model and provider info after creating stream
- Add abort signal tests for all providers
- Update README abort signal section to reflect non-throwing API
- Fix model references in README examples

packages/ai/src/providers/openai-completions.ts

@@ -92,6 +92,8 @@ export class OpenAICompletionsLLM implements LLM<OpenAICompletionsLLMOptions> {
 				signal: options?.signal,
 			});
 
+			options?.onEvent?.({ type: "start", model: this.modelInfo.id, provider: this.modelInfo.provider });
+
 			const blocks: AssistantMessage["content"] = [];
 			let currentBlock: TextContent | ThinkingContent | (ToolCall & { partialArgs?: string }) | null = null;
 			let usage: Usage = {