fix(errors): surface the JSON error detail in REST error messages

[tomershlasky]

Context

The API is adding server-side prompt validation: a prompt over the model's token limit is rejected with HTTP 400 and a JSON body like {"detail": "Prompt is too long: ..."}. This PR makes that rejection (and any other JSON-detail error) read cleanly in SDK error messages.

Changes

New readErrorBody() helper in shared/request.ts, used by process and all three queue request sites. Before:

Processing failed: 400 - {"detail":"Prompt is too long: 362 tokens (maximum 226, including the end-of-sequence token). Please shorten the prompt."}

After:

Processing failed: 400 - Prompt is too long: 362 tokens (maximum 226, including the end-of-sequence token). Please shorten the prompt.

Non-JSON bodies and JSON without a string detail fall through to the raw text unchanged.

Tests

• process and queue.submit with a 400 JSON-detail body reject with the parsed message (msw). Full unit suite: 280 passed; biome clean.
• Note: pnpm typecheck fails on main and on this branch with a pre-existing error in frame-metadata-diagnostics.ts (lookupFrameMetadata missing on RemoteVideoTrack) — unrelated to this change.

[pkg-pr-new]

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npm i https://pkg.pr.new/@decartai/sdk@195

commit: 10cd962