Point the start-image guidance at options.inputSrc and the models that take one

[dazzatronus]

Three places tell people to do something the API now rejects, or describe behaviour the default model does not have.

The ImageToVideoAsset deprecation notice pointed at "VideoAsset with prompt and inputSrc". That was always loose — inputSrc is never a root VideoAsset property, only options.inputSrc — and since seedance stopped accepting a start image it names a migration the default model refuses. It now names shotstack-itv-mini and the right location.

The two VideoAsset descriptions said options includes a starting image, full stop. True for shotstack-itv-mini and the ray models, not for the default.

The smoke test paired seedance-2.0 with options.inputSrc. It passes either way — the generated Zod treats options as an open object — so it was not going to go red, just teach the wrong thing.

Descriptions only; no schema shape changes.

Verify: pnpm build regenerates cleanly and all 71 smoke tests pass. Needs Node 22 locally — this repo has no packageManager pin, so corepack falls through to a system pnpm requiring >=22.13.

https://linear.app/shotstack/issue/ENG-895

[dazzatronus]

Superseded by #133 — same commit on a branch name without internal references.