Context
- This code parses Server-Sent Events (SSE) from the legacy Chat
  Completions streaming API (wire_api = "chat").
- The upstream protocol terminates a stream with a final sentinel
  event: data: [DONE].
- Some of our test stubs/helpers historically end the stream with
  data: DONE (no brackets).

How this was found
- GitHub Actions on Windows failed in codex-app-server integration
  tests with wiremock verification errors (expected multiple POSTs,
  got 1).

Diagnosis
- The job logs included: codex_api::sse::chat: Failed to parse
  ChatCompletions SSE event ... data: DONE.
- eventsource_stream surfaces the sentinel as a normal SSE event; it
  does not automatically close the stream.
- The parser previously attempted to JSON-decode every data: payload.
  The sentinel is not JSON, so we logged and skipped it, then
  continued polling.
- On servers that keep the HTTP connection open after emitting the
  sentinel (notably wiremock on Windows), skipping the sentinel meant
  we never emitted ResponseEvent::Completed.
- Higher layers wait for completion before progressing (emitting
  approval requests and issuing follow-up model calls), so the test
  never reached the subsequent requests and wiremock panicked when its
  expected-call count was not met.

Fix
- Treat both data: [DONE] and data: DONE as explicit end-of-stream
  sentinels.
- When a sentinel is seen, flush any pending assistant/reasoning items
  and emit ResponseEvent::Completed once.

Tests
- Add a regression unit test asserting we complete on the sentinel
  even if the underlying connection is not closed.