feat: provider-level default model options (e.g. OpenAI store)

[cyhhao]

OpenAI Responses store is a per-request option and doesn't fit well in generic provider.options (SDK init options).

Proposal:

• Add provider.<id>.defaults for per-request model options applied to all models of that provider.
• Keep provider.<id>.options for SDK init options (apiKey/baseURL/timeout, etc).
• Migrate legacy provider.<id>.options.store -> provider.<id>.defaults.store automatically.

This enables setting defaults.store once for OpenAI (or other OpenAI-based providers) without repeating per-model options.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [BUG]: Items are not persisted when store is set to false. #8987: [BUG]: Items are not persisted when store is set to false - related to the store option configuration issue
• Try again with store set to true, GPT 5.2 open ai subcription #8288: Try again with store set to true, GPT 5.2 open ai subcription - similar store-related problems with per-request options

Feel free to ignore if none of these address your specific case.

[kcfuler]

encounter the same issue