feat(usage): record streamAborted on mid-stream-death attempts

[kartikkabadi]

Summary

Ports codex-router PR #139's streamAborted metering marker into opencodex: when an upstream stream dies after its 200 head was already committed, the persisted usage attempt must not meter as a success the client never received. The turn is recorded with the synthetic 502 terminal plus streamAborted: true; client cancellations keep opencodex's own 499 client_cancel semantics and never carry the marker.

Changes

• src/usage/log.ts — optional streamAborted?: boolean on PersistedUsageAttempt, whitelisted in normalizeUsageAttempt so only the literal true marker survives the round trip. Backward compatible: ordinary and legacy rows keep their exact JSONL shape.
• src/server/relay.ts — set the marker on both mid-stream read-failure paths:
  • consumeForInspection onReadError (native passthrough): onTerminal("failed", 502) plus streamAborted: true on the active attempt.
  • trackSseForRequestLog read failure (translated responses): incomplete/502 terminal plus the marker, guarded so a cancel-drained pending read never pollutes the 499 row.
• tests/usage-log.test.ts — 4 new tests: marker persistence, backward-compatible omission (undefined/false), legacy rows stay readable, and raw JSONL round trip.
• tests/stream-aborted-marker.test.ts (new) — drives both relay paths end to end through addFinalRequestLog to the persisted JSONL row: mid-stream death meters 502 + streamAborted: true; client cancel meters 499 with no marker.

Verification

• bun test tests/usage-log.test.ts — 34 pass (30 pre-existing + 4 new).
• bun test tests/stream-aborted-marker.test.ts — 4 pass (new file).
• Adjacent relay suites still green: tests/consume-for-inspection-cancel.test.ts, tests/sse-inspector-bounds.test.ts — 34 pass.
• bunx tsc --noEmit — clean for the files in this PR (remaining errors are peers' in-flight edits in other branches' files: src/lib/token-estimate.ts, src/server/responses/empty-completion-guard.ts).

Coordination notes

• src/server/request-log.ts was not modified (TokenCapPort owns it). addFinalRequestLog's existing ...attempt spread carries the marker into the persisted row; the only shared assumption is logCtx.activeAttempt mutation, which relay.ts already uses for transportPhase/terminalSource.
• tests/usage-log.test.ts also carries a small pre-existing working-tree edit to the "persists the rate-limit-429 recovery kind" test (durationMs 4→1, sendCount 2→1, deduped recoveryKinds, usageStatus reported→unreported) that was present before this branch's work and was kept as-is per main-agent direction; it is not authored by this PR.
• Known gap (out of this slice's file ownership): the eager relay path (relay-eager.ts onSynthetic "failed", wired in responses/core.ts) synthesizes the same 502 terminal but does not yet set streamAborted. Follow-up recommended.

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