Add top-level `ResponsesClient` factory on `OpenAIClient` (#1143)

* Add top-level responses client factory

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* Validate responses client factory change

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* Remove unrelated csproj newline change

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* Copy shared pipeline options to responses client

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* Add pipeline-only responses factory coverage

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* Use non-routable URI in responses smoke test

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* Reuse shared pipeline for responses client

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* Copy responses option properties from top-level client

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* Document TopLevelResponsesClient purpose

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* Trim responses factory options

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* Regenerate API listings

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