fix(api): keep the images wrapper on combo routes and default Codex to b64_json (#12362)

POST /v1/images/generations through a combo returned a bare array instead of the OpenAI {created, data} payload: executeImageCombo() unwrapped one level too many, and the n used for cost calculation read the same double-nested shape, so it was always 0. The combo path now returns the handler payload unchanged, matching the direct-model path.

Second half: Codex image results emitted a data: URI in url whenever response_format was not b64_json, but OpenAI returns b64_json for the gpt-image-* family — clients that omit the field, Codex CLI's built-in image_gen among them, could decode neither shape. Codex now defaults to b64_json; an explicit response_format: "url" keeps its previous behaviour. Both land together because fixing one leaves Codex CLI failing at the other.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.