fix(core): show structured provider error details

[Dante-dan]

Issue for this PR

Refs #52042

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When the AI SDK supplies a generic HTTP error, show a decoded provider message from either a structured error object or a plain string error in SDK data or a JSON response body. Preserve the original response body and keep the SDK message and status/code fallbacks when there is no provider message. For example, {"error":"The selected model is not multimodal"} now displays that message instead of Provider request failed with HTTP 400.

This changes error reporting only. It does not change custom-model image capabilities or prevent rejected images from recurring in a session. The raw response shape from #52042 has not been captured, so the exact reported provider response remains unconfirmed.

How did you verify your code works?

• cd packages/core && bun test test/aisdk.test.ts — 46 passed, including string errors from SDK data and the JSON response body.
• cd packages/core && bun typecheck — passed.
• bun run check — lint passed; repository typecheck failed in unmodified packages/session-ui files with implicit-any and generic-type errors. I have not established whether that failure also occurs on the target branch.
• git diff --check — passed.
• An earlier full core test run was stopped after unrelated LocationServiceMap timeouts; it did not complete and was not rerun for this commit.

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[Dante-dan]

This PR addresses only the structured provider-error message part of #52042. The custom-model image capability and session recovery questions remain open. I used Refs #52042 because Fixes or Closes would automatically close the whole issue when this partial fix merges. Could a maintainer accept the reference for this scoped PR and remove needs:issue, or point me to a more precise existing issue to link?