The useMessagesApi branch of OpenAIEndpoint.createRequestBody never called
_applyReasoningEffort, so a custom endpoint declaring supportsReasoningEffort
never received the selected effort. Forward it like the Responses and Chat
Completions branches, and add a 'messages' reasoningEffortFormat that writes
the Anthropic Messages shape output_config.effort (the default on /messages,
overridable via reasoningEffortFormat). The scrub merges into output_config
instead of replacing it so sibling fields (e.g. structured output format)
survive.
Fixes#325121
Dave
ac0fb39943