fix(rest/nodejs): answer validation failures with the UCP error envelope

[vishkaty]

What

A request rejected by payload validation draws a plain text 422 built from the raw zod issues. A validation failure is the only rejection this server still answers as plain text, and the checkout error taxonomy has answered with the UCP envelope since #174. checkout-rest.md gives protocol errors one shape, a JSON body carrying code and content, so a platform can act on the rejection.

POST /checkout-sessions
{"line_items": "not-an-array"}

HTTP 422  content-type: text/plain
✖ Expected array, received string
  → at line_items

Fix

prettyValidation keeps building the same readable diagnostic and now renders it through the existing ucpErrorResponse helper as INVALID_REQUEST with status 422, so the envelope carries the diagnostic as content. Log output is unchanged.

Testing

• New test/validation_envelope.test.ts: a validation failure must answer application/json in the UCP envelope shape with the offending member named in content, and a valid create still returns 201. The envelope test fails on main and passes with the fix; the suite is 154 passing with it.
• Sweep: a schema guided sweep of 261 request mutations drew 122 plain text rejections on main and zero after this change, with no other bucket moving.

[carolinerg1]

Hi @vishkaty, thank you for submitting this fix. This is minor, just formatting feedback, but it looks like it's failing Lint test - could you please take a look at "return ucpErrorResponse" and split the arguments across multiple lines.

[vishkaty]

Done, split in bdbf45a and the pinned pre-commit now passes on both files. Thank you for the review.