test(gateway): correct the Anthropic SSE stub to the shape the API emits

[ophiocus]

What changes

anthropicSseBody() and the abort-stream stub now emit the frames api.anthropic.com actually sends:
usage at the top level of message_delta, as a sibling of delta, with delta carrying only the stop
fields; a full Message envelope on message_start; and the null cache counters an uncached request
reports.

No production code changes. The suite passes unchanged.

Why

The stub is what every Anthropic streaming assertion in gateway.test.ts is measured against, and it
encodes a frame the provider never sends. usage is a sibling of delta — the shape is typed in a
dependency this repo already has, @anthropic-ai/sdk, resources/messages/messages.d.ts:

export interface RawMessageDeltaEvent {
    delta: RawMessageDeltaEvent.Delta;   // container, stop_details, stop_reason, stop_sequence
    type: 'message_delta';
    usage: MessageDeltaUsage;            // <- sibling of delta, not inside it
}

That matters for what the tests can catch. UsageTee reads usage from the top-level parsed object and
from parsed.delta; on the real wire the first is what meters every streamed output token and the second
never fires. With the old stub the coverage is inverted — mutating services/gateway/src/usage.ts on
main:

mutation	old stub	corrected stub
drop the parsed.delta read — dead against the real wire	streaming assertions fail	unaffected
drop the top-level parsed read — meters real streamed output	streaming assertions still pass	fails

So the suite pinned dead code and left the live metering path unpinned. Cost from that path lands in
spend_counters and is enforced by hardBudgetBlock, which is why it seemed worth correcting rather than
leaving.

The shape has been in the stub since a6553e9 (2020-07-03), so this is independent of any open work. PR #148
copies the same shape into new unit tests; I have left the detail on that thread rather than touching it
here, since its production fix is correct.

Verification

pnpm --filter @facility/gateway test     # 52 passed, unchanged by this PR
pnpm exec biome check services/gateway/test/gateway.test.ts

Beyond the suite: the mutation table above, run both ways on main. Postgres from
docker-compose.dev.yml; without it the DB-backed cases skip and the count drops to 20 passed | 1 skipped.

• pnpm verify passes locally — not run. Gateway package suite + biome only. Say the word if you want the full run before review.
• Behaviour verified beyond the test suite — the two mutations above.
• Documentation updated, or no user-facing change — no user-facing change.

[adrian-lorenzo]

Thanks for the contribution!

Since #148 has now landed, please update this branch and also correct the MESSAGE_DELTA fixture in services/gateway/test/usage.test.ts, plus the comment in services/gateway/src/usage.ts that still says output usage is under delta.usage. Otherwise the integration stub will match Anthropic’s wire format while the new unit fixtures and production documentation continue to encode the old shape.

[ophiocus]

Since #148 has now landed, please update this branch and also correct the MESSAGE_DELTA fixture in services/gateway/test/usage.test.ts, plus the comment in services/gateway/src/usage.ts that still says output usage is under delta.usage. Otherwise the integration stub will match Anthropic’s wire format while the new unit fixtures and production documentation continue to encode the old shape.

Done — and since this is the branch's first update: it is updated by merge, not rebase, so the commit you reviewed stays as it was.

• 880fa82 — main merged in (brings fix(gateway): meter Anthropic streaming input tokens from message_start #148 and fix(guards): resolve markdown links against a normalised repository root #192).
• e248458 — the MESSAGE_DELTA unit fixture now emits the real frame (usage as a sibling of delta, with delta carrying only the stop fields), and the usage.ts comment documents that shape. The parsed.delta merge source is deliberately kept as tolerance for relays that nest usage inside the delta, and the comment now says so explicitly, so it can no longer read as a description of the wire format.

Gateway suite 57/57 against facility_gw; biome and typecheck clean.

[adrian-lorenzo]

Thanks for the solid contribution!