fix(codex): handle Responses stream failures

[nguyenha935]

Summary

• detect retryable OpenAI Responses SSE failures returned inside HTTP 200 before client-visible output, so combos can fall back to the next model/account
• avoid retrying after Responses output has already started, because fallback is no longer safe once the client has seen stream content
• mark late terminal Responses failures as failed request details/account failures instead of recording them as successful streams

Tests

• npm --prefix tests test -- unit/stream-complete-callback.test.js unit/stream-terminal-outcome.test.js unit/responses-semantic-failure.test.js unit/codex-sse-capacity.test.js unit/combo-semantic-failure.test.js
• npm run build

Local runtime verification on tiny

• Applied temporary production patch under /opt/9router without resetting existing local patches.
• Ran existing production targeted tests: unit/responses-semantic-failure.test.js, unit/codex-sse-capacity.test.js, unit/combo-semantic-failure.test.js.
• Built /opt/9router, refreshed standalone runtime links, restarted 9router.
• Verified service active, /login 200, /providers/gemini.png 200, and a Next static asset 200.