Declares OpenAI's provider-executed Responses `web_search` built-in in place of
the client-side `web_search` function, mirroring the existing xAI native-search
path. Selected via `web.search_backend: openai-native`; search-only, so
`web_extract` keeps resolving to its own backend.
The Responses adapter already recognises built-in tool types
(`_RESPONSES_BUILTIN_TOOL_TYPES`) and preflight passes them through, so the only
missing piece was the swap itself plus a provider name for the config to point at.
Gating is deliberate: two-sided (Codex backend AND a selected openai-native
backend) and fail-closed, so a custom OpenAI-compatible endpoint or an
unresolved provider leaves the client tool untouched.
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