Extract recursive tool loop into TextGenerationLoop and convert OpenAI

[vinitkadam03]

Edited by @pushpak1300

Currently every gateway reimplements the same recursive multi-step tool loop. If you want to add a new provider, you have to copy orchestration logic (tool dispatch, max-step enforcement, step aggregation, response assembly) in addition to the actual request and response mapping.

This PR introduces TextGenerationLoop as a centralized orchestrator and converts OpenAI and Azure OpenAI to single-turn gateways. Gateways now only build one request, parse one response, and yield one stream turn. The loop handles everything else.

Before, OpenAI's ParsesTextResponses trait contained a processResponse method that recursively called itself after executing tools, building the final TextResponse inline. The same pattern was duplicated across providers.

After this PR, the gateway looks like this:

// Gateway implements SingleTurnTextGateway
public function generateSingleTurn(...): SingleTurnResponse
{
    $body = $this->buildSingleTurnBody(...);
    $response = $this->client->post('responses', $body);
    return $this->parseTextResponse($response->json(), $provider, filled($schema));
}

// generateText delegates to the loop
public function generateText(...): TextResponse
{
    return $this->buildTextGenerationLoop()->generate(...);
}

TextGenerationLoop owns tool execution, max steps, step accumulation, and final response assembly. SingleTurnTextGateway is an opt-in sibling contract to the existing TextGateway, so unconverted providers keep working while we migrate one at a time.

StreamEnd now carries an optional responseId so the loop can chain OpenAI's previous_response_id across turns. It works, but it couples a generic event to a provider-specific detail. WDYT about moving this to a side channel instead?

The buildTextGenerationLoop() helper is duplicated between OpenAI and Azure. It should probably become a shared trait once one more gateway is converted.

[vinitkadam03]

Hey @pushpak1300, following up on #349. Per your suggestion about cherry-picking rather than merging everything in one go, I took a stab at a smaller foundation-only path: this PR ships StepLoop + the new contract with just OpenAI (and Azure, since they share traits) converted. Every other gateway is untouched.

Kept it in draft for now; no rush, just trying to help. PR2 for the Anthropic migration is ready to stack on this once the approach is confirmed.

Honest open questions called out in the description (StreamEnd carrier, SingleTurnTextGateway as public contract, StepContext bag pattern). Happy to iterate on any of them.

[pushpak1300]

• Renamed StepLoop to TextGenerationLoop so the name is clearer.
• generateText() / streamText() still exist and now delegate through the loop.
• Fixed the streaming behavior so intermediate provider-turn StreamEnd events are not exposed before the tool loop finishes.
• Fixed max-step behavior so tools are not executed on the final allowed step when their results cannot be sent back to the provider.
• Added focused tests for both regressions.