fix(rest/python): synchronize simulated shipments with remaining quantities

[arjun2075]

POST /testing/simulate-shipping/{id} appends a shipped fulfillment event whose per-line quantity is the quantity "fulfilled in this event" (2026-04-08 fulfillment_event.json), but leaves quantity.fulfilled = 0 and status = "processing" — the stored order contradicts its own event log, and the documented status derivation computes "processing" for an order the events say is shipped.

This change defines the simulation as "ship all remaining units": each call ships every not-yet-fulfilled unit, so the event's per-line quantities are exactly the fulfilled increments for that call. A call with nothing remaining appends no event and invents no quantities; it still normalizes zero-total lines to removed (the change is persisted, no shipment event is emitted). Zero-total lines are excluded from the event — the April schema requires each event quantity >= 1 — and recorded as removed.

Version-applicability note: no 2026-04-08 requirement governs counter/event consistency for this simulation path — this is a reference-implementation self-consistency fix, not spec enforcement. No schema, endpoint, or protocol-contract change.

Tests: five new cases (fresh order, prior partial fulfillment, repeated call, mixed removed/active lines, all-zero-total order) fail on the base and pass with the fix; full integration_test.py is green (43 passed).

[damaz91]

Thanks @arjun2075! Great catch synchronizing the simulated shipping increments with the unfulfilled quantities and line-item status derivation, and thorough test coverage. (Pushed a quick EOF newline fix so the pre-commit linter passes.)