docs(api): mark coach done.usage.model optional to match runtime

The streaming-doc description listed `model` as a required field of the
`done.usage` envelope, but the runtime CoachUsage type has it optional /
nullable. Align the description and regenerate the OpenAPI contract.

docs/api/openapi.yaml

@@ -4152,7 +4152,7 @@ paths:
         (a cadence-suggestion card: `{ type, suggestion }`), `reasoning` (v1.18.9, optional reasoning-summary text: `{
         type, text }` — emitted only by reasoning-capable providers; absent otherwise), `done` (`{ type, conversationId,
         messageId, usage? }` — v1.18.9 adds the optional `usage` envelope `{ totalTokens, promptTokens?,
-        completionTokens?, model }`, server-authoritative; clients display it, never recompute), or `error` (`{ type,
+        completionTokens?, model? }`, server-authoritative; clients display it, never recompute), or `error` (`{ type,
         code, message }`). The HTTP status is 200 even for a provider/refusal outcome — clients dispatch on the `error`
         frame, not the status. Clients ignore unknown frame types (additive evolution). Omitting `conversationId` starts
         a new conversation. Coach-gated; budget- and rate-limited. Auth via cookie or Bearer."