Submit dispute evidence to the connector

Finalizes the attached dispute evidence and submits the response to the connector for review (`POST /v1/disputes/:id/submit`). Distinct from attaching evidence (`POST /api/disputes/{id}/evidence`): this commits the response and stamps `lastPlatformActionAt` on the MVS dispute-extension sidecar. Pass an `Idempotency-Key` header to make submission safe to retry. The engine's response body is returned verbatim. Note: requires the `disputes:write` scope, which is defined in the scope catalog but is NOT included in the default API-key grant — keys must be explicitly granted `disputes:write`.