feat(websocket): accept official response.steer events

[songlairui]

Summary

• /v1/responses WebSocket previously treated response.steer as an unknown type and returned unsupported websocket request type: response.steer, so official mid-turn steering never reached Codex/xAI.
• Accept the official three-field payload (type, previous_response_id, input) without injecting stream or model, and leave the last response.create snapshot unchanged.
• Codex/xAI websocket executors now clone steer frames instead of rewriting them to response.create. HTTP fallback cannot mid-turn-steer and returns steering_not_supported.
• Concurrent read-while-generating (true in-flight interrupt) is left as a follow-up; official clients already allow the original response to complete first, then continue.

Test plan

• go test ./sdk/api/handlers/openai/ -run 'TestNormalizeResponsesWebsocketRequestSteer|TestNormalizeResponsesWebsocketPassthroughRequestSteer|TestNormalizeResponsesWebsocketRequestSteerRejectsEmptyInput|TestNormalizeResponsesWebsocketRequestStillRejectsUnknownType|TestResponsesWebsocketRequestRequiresCurrentUpstream|TestNormalizeResponsesWebsocketRequestCreate'
• go test ./internal/runtime/executor/ -run 'TestBuildCodexWebsocketRequestBodyPreserves|TestBuildXAIWebsocketRequestBodyPreservesSteer|TestBuildXAIWebsocketRequestBodySetsStore'
• Against a Codex Responses WS session: send official response.steer after response.create and confirm it is forwarded instead of rejected as an unknown type
• Confirm HTTP fallback still returns steering_not_supported
• Confirm unknown types such as response.cancel are still rejected

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