### What happened?

OpenAI Responses cost tracking can bill a request at priority rates even when the provider response says the request was served as `default`.

This is a cost-calculation precedence bug for `/v1/responses`, not a UI configuration issue.

The problematic case is:

- the request asks for `service_tier="priority"` - the OpenAI Responses response contains `service_tier="default"` - LiteLLM cost calculation prefers the request-level tier before the response tier

That means cost tracking can select `input_cost_per_token_priority` / `output_cost_per_token_priority` even though the served tier reported by the provider was `default`.

Expected behavior: when a provider reports the served tier on the response, cost tracking should prefer that concrete served tier over the request preference. A response-level `default` should map to standard/default pricing.

### Steps to reproduce

```python import litellm from litellm import completion_cost from litellm.types.llms.openai import ResponsesAPIResponse

model = "test-openai-responses-default-tier-cost-model" litellm.register_model( model_cost={ model: { "input_cost_per_token": 0.001, "outp…