[Docs] Explain OpenAI-compatible custom request fields

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​Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

The OpenAI-compatible provider guide now documents the Extra Body setting, includes a Sail Research metadata.completion_window example, explains protected fields including response format and profile-export security, and shows the complete unclipped VS Code settings control.

Why this change was made

Users need a concrete, provider-focused example for configuring custom request-body fields without relying on a proxy or mistaking them for HTTP headers.

Impact

OpenAI-compatible provider users can follow a complete setup example for custom request metadata and understand the validation and security boundaries before saving a profile.

Related PRs

• Zoo-Code implementation: [Feat] Add custom request fields for OpenAI-compatible providers Zoo-Code#1350

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No documentation mismatches outstanding.

• Update the protected-field guidance to include response_format and avoid implying every reserved field has a dedicated setting (docs/providers/openai-compatible.md:70).

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