fix(responses): handle response.done terminal events

[rifuki]

Summary

• treat response.done as a terminal OpenAI Responses stream event
• keep emitting the OpenAI data: [DONE] sentinel for same-format Responses passthrough streams
• add a regression test for response.done

Why

Codex/OpenAI Responses streams can terminate with:

event: response.done
data: {"type":"response.done", ...}

9router already recognizes response.done in some older response conversion paths, but the current Responses passthrough terminal-event helper only accepts response.completed, response.failed, and error. When upstream sends response.done, the stream can be treated as incomplete and a synthetic response.failed may be emitted, which causes clients to surface errors like stream closed before response.completed / stream disconnected before completion.

This patch makes the same-format Responses passthrough path accept response.done as terminal, and restores the [DONE] sentinel for Responses passthrough streams so OpenAI-compatible SSE clients see a clean stream ending.

Related scan

I checked the current open PRs/issues before opening this:

• fix(codex): handle Responses stream failures #2089 is related to retryable/semantic Responses stream failures, but does not add response.done to the terminal event set and is currently conflicting.
• fix(responses): forward usage as input_tokens on response.completed #2082 and fix(responses): default text.format for openai-compatible responses providers (#2093) #2112 cover Responses usage/text-format issues, not stream termination.
• fix: update codex integration to 1.136-compatible headers and response events #1618 previously mentioned response.done, but it was closed without merge and the current master terminal helper still does not include it.
• Fail incomplete Responses streams explicitly #1648 introduced the incomplete Responses stream test coverage, but was also closed without merge.

Tests

./node_modules/.bin/vitest run --config tests/vitest.config.js \
  tests/unit/openai-responses-terminal-event.test.js \
  tests/unit/responses-abort-terminal.test.js \
  tests/unit/headroom-responses-format.test.js \
  tests/unit/codex-tool-normalization.test.js

Result: 4 files passed, 11 tests passed.

[decolua]

Thanks @rifuki! Reviewed and accepted — response.done correctly added to the Responses passthrough terminal-event set, and the data: [DONE] sentinel is restored with proper dedupe guards. Merging into master locally.