fix(sse): emit trailing usage estimate in translate streams when upstream stays silent

[maxmad64bis]

⚠️ base-red inherited: #12732

Summary

Chat clients talking to a Responses upstream never got token counts. #12151 covered only one of the two stream paths: it emits the fallback estimate as a trailing usage-only chunk in passthrough mode, but translate mode computed the estimate after closing the stream, so it only ever reached the server log. This adds the missing trailing chunk in the translate flush. Real upstream usage still arrives inside its own finish chunk and stays untouched — no second chunk in that case.

Related Issues

• Related to fix(sse): estimate usage in passthrough stream even with include_usage #12151, fix(release): drain the 2026-09-01 base-red window — passthrough usage regression + radarPage i18n keys #12327

Validation

• Change type: other
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Focused suites, all green on upstream/release/v3.8.51 after the change: stream-translate-usage-trailing (3/3, new), stream-utils (52/52), stream-passthrough-usage-estimation + stream-numeric-ids + empty-choices-no-inject + responses-usage-trailing-6906 + responses-translation-fixes (64/64), plus the commentary/control-lines/usage companions (18/18) and the Responses commentary set (67/67 with the numeric-ids group). typecheck:core and typecheck:noimplicit:core clean. npm run lint exits 2 on this base, and it does so without reporting a single rule violation — the only output is There are suppressions left that do not occur anymore, i.e. stale entries in config/quality/eslint-suppressions.json elsewhere in the tree. Linting open-sse/utils/stream.ts on its own returns exactly the 2 no-unused-vars errors already frozen for that file, so this change adds none. I left the box unchecked rather than tick it on a run that exits non-zero. No VPS gate applies — it's local stream assembly, no upstream round-trip involved.

Tests Added Or Updated

• tests/unit/stream-translate-usage-trailing.test.ts (new): silent Responses upstream in translate mode asserts exactly one choices: [] chunk carrying usage before data: [DONE] with comments disabled; real response.completed usage asserts verbatim single delivery with no estimated marker; passthrough silent-upstream behavior pinned unchanged.

Coverage Notes

• Change is in open-sse/utils/stream.ts (translate flush + one latch in the translated-item emitter) plus open-sse/utils/usageOnlyChunk.ts, the chunk builder now shared with the passthrough flush. The three new cases execute the added branch both ways (silent upstream emits, real usage suppresses) plus the untouched passthrough path.

Reviewer Notes

• open-sse/utils/stream.ts: a translateForwardedUsage latch mirrors the existing passthrough guard, the estimate moved ahead of the [DONE] emission, and the trailing chunk reuses filterUsageForFormat + timing.withTps like passthrough. It's gated to OpenAI-chat-shaped clients (sourceFormat === FORMATS.OPENAI with the [DONE] terminator): Responses/Claude clients terminate on their own protocol events. One subtlety found while verifying: when a translator already merged an estimate into the finish chunk (Claude upstream), the latch is set there so the flush doesn't append a second chunk.
• stream.ts is a frozen file: the chunk builder moved out, and the remaining flush wiring (+15 lines over the base) is rebaselined in config/quality/file-size-baseline.json with a note, like the existing _rebaseline_* entries.