Widen the #65991 single-writer fence to run_codex_stream: each codex
attempt claims the delta sink before consuming events, and the consume
loop's interrupt_check now also stops the instant a newer attempt
supersedes this one. Parity with the chat_completions / anthropic /
bedrock paths from the salvaged fix.
Two regression tests: superseded codex stream is fenced mid-stream;
sole-writer codex stream delivers unchanged.
Assert that a superseded stream (older writer token, other thread) is fenced
from the delta sink, the active writer is never fenced, a non-claiming thread
is never treated as a writer, and the real consume loop stops the instant it is
superseded — so two streams can never interleave into one turn (#65991).
Teknium
Alex Fournier
Jeffrey Quesnelle
HexLab98