fix(gateway): keep hard-budget reservations when stream usage is incomplete

[o-mid]

Summary

Fixes #248.

When a streamed request is interrupted after Anthropic reports partial usage (for example message_start input tokens) but before the terminal message_delta, the gateway was reconciling hard-budget reservations down to that partial measured cost. That undercounts spend because the provider may have charged for additional generation whose usage never arrived.

This change tracks whether provider usage telemetry is complete separately from whether any usage was observed, and keeps hard-budget reconciliation conservative when usage is incomplete.

Changes

• UsageTee exposes usageComplete, set when a terminal usage-bearing event is parsed
• RequestRecord carries usageComplete through the proxy pipeline
• resolveMeteredCost() falls back to max(measured, estimated) when a provider-charged error has incomplete usage
• Integration test 9c reproduces client abort after message_start only and asserts the hard-budget counter stays at the reservation

Test plan

• pnpm --filter @facility/core build && pnpm --filter @facility/db build && pnpm --filter @facility/gateway test
• New unit tests for UsageTee.usageComplete and resolveMeteredCost
• Existing client-abort test still passes for terminal message_delta partial usage

[adrian-lorenzo]

Thanks for the contribution!

usageComplete protects the client-abort path, but resolveMeteredCost only applies the conservative fallback when record.status === "error". A 200 stream can end cleanly or emit an in-band provider error before the terminal usage frame; that leaves usageComplete false while the status remains ok, and the hard-budget reservation is still reconciled down to partial measured spend.

Please make incomplete provider telemetry conservative regardless of the transport status, and add unit and integration coverage for a 200 stream ending after message_start without message_delta. The changed files also need formatting with Biome.

Once those cases are covered, this should be ready to approve.

[o-mid]

Addressed review: resolveMeteredCost now applies the conservative fallback whenever providerMayHaveCharged and usage is incomplete, regardless of HTTP status. Added test 9e for a 200 stream that ends after message_start without message_delta, plus a unit test for the ok/incomplete case. Ran Biome on changed files.