fix(ai): show provider error bodies when no message is recognized - #51978

Problem

#42610 removed the executor's redaction logic, but it also removed the raw-body fallback. When a provider's error body has no error.message or top-level message, users now see only Provider request failed with HTTP N, even though the provider explained the problem.

Live examples that currently show only the generic message: Mistral {"detail":"Invalid API Key"}, NVIDIA {"title":"Gone","detail":"The model ... has reached its end of life ..."}, xAI {"code":"invalid-argument","error":"Incorrect API key provided..."}, Cloudflare {"errors":[{"message":"Authentication error"}]}.

Fix

In packages/ai/src/route/executor.ts, without a recognized message, show the status and the raw body, capped at 2,000 characters:

Provider request failed with HTTP 401: {"detail":"Invalid API Key"}

• The body is not redacted or otherwise rewritten, only truncated.
• HTML error pages from gateways and proxies, and empty bodies, keep the generic message, as in v1.
• reason.body still holds the complete body, and classification and retries are unchanged.

Recognizing more provider message layouts and fixing classification gaps will come in follow-up PRs.

Tests

• New tests cover an unrecognized JSON body, truncation, and HTML pages.
• Two existing expectations change: plain-text and blank-message bodies now include the body.
• bun test in packages/ai and bun run check pass.