feat(api): add onSuccess handler support (Addresses #566)

[Arkitecht]

Description

Adds fluent onSuccess method to add handlers for when the response is successfully received by the LLM. Sends the request and response as arguments. Can assign multiple handlers per-request.

For example:

$response = Prism::text()
            ->using(Provider::Anthropic, 'claude-3-5-sonnet-latest')
            ->onSuccess(function ($request, $response): void {
                //AiUsage is a model that stores token usage
                AiUsage::create([
                     'provider' => $request->provider(),
                     'model' => $request->model(),
                     'prompt_tokens' => $response->usage->promptTokens,
                     'completion_tokens' => $response->usage->completionTokens
                ]);
            })
            ->onSuccess(fn($request, $response) => dump('Success!'))
            ->withPrompt('Who are you?')
            ->asText();

[sixlive]

I LOVE THIS, as part of #601 I also added an onComplete handler system. So I'm going to close this in favor of that but this is a banger PR. Thank you so much!