feat: allow provider-level store option

[cyhhao]

Background

Provider options currently support baseURL/apiKey and setCacheKey, but there is no way to set the OpenAI Responses store flag at the provider level. This makes users repeat store=false for every model.

Changes

• Read provider.options.store in ProviderTransform.options and emit it into request options.
• Add store to the provider options schema so it is documented and validated.

Verification

• bun test test/config/config.test.ts