Add helper for response created SSE events in tests

[pakrym-oai]

Summary

• add a reusable ev_response_created helper that builds response.created SSE events for integration tests
• update the exec and core integration suites to use the new helper instead of repeating manual JSON literals
• keep the streaming fixtures consistent by relying on the shared helper in every touched test

Testing

• just fmt

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https://chatgpt.com/codex/tasks/task_i_68e1fe885bb883208aafffb94218da61