fix: normalize responses streaming terminal output

[ttt132]

Fixes #2819

Related: #2838

Summary

• normalize terminal Responses SSE events when response.output is null, missing, non-array, or empty while accumulated output exists
• rebuild terminal response.output from streamed deltas and image output items when available
• fall back to response.output: [] so OpenAI SDK clients can iterate terminal output safely
• preserve response.failed events without adding synthetic output

Difference from #2838

• fix(openai): normalize terminal responses output #2838 normalizes null/missing terminal output to an empty array
• this PR preserves final output content when prior streaming deltas are available, then falls back to an empty array only when there is no reconstructable output

Tests

• GOPROXY=https://goproxy.cn,direct /tmp/go1.26.3/bin/go test ./internal/service -run 'TestOpenAIStreamingNormalizesTerminalOutput' -count=1
• GOPROXY=https://goproxy.cn,direct /tmp/go1.26.3/bin/go test ./internal/service -run 'TestOpenAIStreaming|TestHandleNonStreamingResponse_APIKeyFallsBackToSSEBodyWhenContentTypeIsWrong|TestHandleSSEToJSON_ReconstructsImageGenerationOutputItemDone' -count=1
• GOPROXY=https://goproxy.cn,direct /tmp/go1.26.3/bin/go test ./internal/service -count=1