Case B: when a slot wait flushes SSE ping comments first (Writer.Written
becomes true), the previous ensureForwardErrorResponse short-circuited
on `c.Writer.Written()` and returned false without notifying the client.
Subsequent upstream errors (http2 timeout, stream INTERNAL_ERROR, etc.)
produced silent EOF; Codex CLI reported "stream closed before
response.completed" just like the user-slot timeout case.
Remove the Written() early return; coerce streamStarted to true when
Writer has already been written to, and let handleStreamingAwareError
walk the existing logic — which now (thanks to the previous commits)
emits a protocol-compliant response.failed for /responses paths and the
legacy `event: error` for others.
Update tests that previously asserted "do not override written response":
the new contract is to *append* an SSE terminal frame so the client sees
a clean close instead of EOF. recoverResponsesPanic inherits this fix.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
b34cc71bee