fix(stream): fence previews by model request

[yanpgwang]

Summary

• correlate private preview envelopes with their durable model-request span
• prevent delayed frames from a failed or interrupted request from leaking after a later request starts
• preserve the public SSE payload and retain a fallback for older publishers

Verification

• make test-service
• go test -race ./...
• go vet ./...
• make lint LINT_BASE=origin/main

Part of #58.