fix(types): make downloadImage opts optional

[Yahiro025]

Summary

Make opts optional on Plotly.downloadImage so the TypeScript signature matches runtime behavior and Plotly.toImage.

Fixes #8072

Why

src/snapshot/download.js does opts = opts || {}, so Plotly.downloadImage(gd) is valid at runtime. After #8066, toImage marks opts optional, but downloadImage still required it and broke type-checking for the one-argument call.

Change

In src/types/core/api.d.ts:

export function downloadImage(root: RootOrData, opts?: DownloadImgopts): Promise<string>;

Types only. No runtime change.

Tests

Types-only change. No jasmine suite covers .d.ts signatures. Runtime one-arg behavior is already covered by existing download tests and by opts = opts || {} in src/snapshot/download.js.

Checklist notes

• Issue [BUG]: incorrect downloadImage type definition #8072 is open, labeled bug, not plotly-internal
• No competing open PR for this issue
• Draftlog placeholder draftlogs/XXXX_fix.md will be renamed to the PR number in a follow-up commit
• 🐦

[camdecoster]

Thanks for the PR! Could you please remove the draftlog? We don't need one for such a small update.

[Yahiro025]

@camdecoster Removed the draftlog as requested — typings-only change remains.