feat(config): provider requestOptions for request-level params

[cyhhao]

Fixes #9028.

Context

• store is a per-request OpenAI Responses option; putting it under generic provider.options (SDK init options) is awkward.
• Introduce a clear separation:
  • provider.<id>.options: SDK init / transport settings (apiKey, baseURL, timeout, ...)
  • provider.<id>.requestOptions: default per-request model options applied to all models in that provider.

Changes

• Add provider.<id>.requestOptions and plumb into ProviderTransform.options().
• Apply requestOptions.store only for OpenAI providers (OpenAI still defaults to store=false).
• Migration: if users had provider.<id>.options.store, it is moved to provider.<id>.requestOptions.store during config load.

Example

{
  "provider": {
    "openai": {
      "requestOptions": { "store": true }
    }
  }
}