[FEATURE]: Support configuring OpenAI `service_tier='flex'` for chat/completions and document in config

[duysqubix]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I am using opencode with OpenAI and want to be able to select the service_tier='flex' option for supported OpenAI models to reduce costs. At present, service_tier is propagated for Responses API, but not for chat/completions requests, and configuration/docs do not clearly document/validate this option.

Enhancement Proposed:

• Add serviceTier to the OpenAI Responses provider options schema to ensure configuration is typed and documented.
• Add serviceTier to the OpenAI-compatible chat provider options schema and propagate as service_tier in the chat request body (in openai-compatible-chat-language-model.ts).
• Update docs (e.g., models.mdx) to illustrate how to use serviceTier: "flex" in opencode.json.
• Optionally update/add tests to assert that chat and response bodies include service_tier when configured.

Benefits:

• Full support for OpenAI flexible service tiers (flex and priority) for both response and chat endpoints.
• Enables users to control cost via configuration for supported models (gpt-5, o3, o4-mini, etc).

[github-actions]

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

• [FEATURE]: Support OpenAI service tier (flex/priority processing) #7511: [FEATURE]: Support OpenAI service tier (flex/priority processing) - directly requests the same feature
• [FEATURE]: Cost calculation according to service_tier (OpenAI) #11597: [FEATURE]: Cost calculation according to service_tier (OpenAI) - specifically addresses configuring serviceTier in opencode.json and cost calculations

Feel free to ignore if these don't address your specific case.

[duysqubix]

There are dups found for this.. closing.. Thanks