Coerce missing billable_units to null on MCP run payloads

[eliteprox]

Summary

billable_units is forwarded from result.billableUnits without coercion in lib/runs/execute.ts. When that value is undefined (non-blocking/queued executions or older gateway responses), JSON.stringify omits the key even though the intended contract is number | null and the field should always be present.

Caught on main by a follow-up review of code that has not changed since the last PR review.

Location

lib/runs/execute.ts (~line 252):

billable_units: result.billableUnits,

Contract tests already assert the field is present as number | null:

• tests/contracts/run-execution.test.ts — "never repeats paid execution when terminal storage fails" expects billable_units to be null
• "forwards billable_units from the gateway result" expects a numeric value when the gateway provides one

There is no coverage for the missing/undefined case.

Proposed fix

Coerce missing units to null before putting them on the payload, e.g. result.billableUnits ?? null.

Add a contract test where the gateway result omits billableUnits (queued / non-blocking / older response) and assert the JSON payload still includes "billable_units": null.

Test plan

• Coerce undefined / omitted billableUnits to null in the execute payload
• Keep forwarding numeric values unchanged
• Add a contract test for the omitted-units path