[touch2be]

Problem

A Claude Code run can fail with a completely opaque message:

Model provider request failed
detail: Claude Code request failed: invalid_request
code: unknown   retryable: false

invalid_request is one of the Claude Agent SDK's SDKAssistantMessageError
codes (Anthropic HTTP 400). The SDK hands us only the bare code — no detail.
On OpenGUI's side two gaps turned it into a dead end:

1. assistantErrorMessage had no case for invalid_request (nor
   model_not_found, account_on_hold, verification_required,
   cloud_credential_error), so it fell through to the generic wording.
2. A ClaudeCodeError carries no numeric status, and normalizeModelError
   had no text rule for a plainly-worded bad request — so it landed in
   unknown / "Model provider request failed", with retryable: false and
   nothing actionable for the user.

Real-world trigger that surfaced this: a long session kept re-sending image
attachments whose files had disappeared from /tmp, so every subsequent turn
got a 400 and the session was permanently wedged behind a meaningless error.

Change

• protocol.ts — assistantErrorMessage: cover every SDK error code.
  invalid_request now explains the likely causes (request too large, or a
  referenced attachment that can no longer be read) and suggests starting a
  new session / removing missing attachments; model_not_found is worded
  clearly; the account/verification/cloud-credential codes map to auth.
• transport.ts — normalizeModelError: classify plainly-worded bad
  requests (invalid request / model not found) as invalid_request even
  without a numeric status, so Claude Code's text-only errors reach the right
  bucket. Narrow, additive regex; benefits any provider that words a 400 in
  text.
• Tests for both the errorMessage text path and the new
  assistantErrorMessage code path.

Testing

vp test packages/harness/src/models/claude-code/protocol.test.ts   # 25 passed
vp test .../transport-contracts.test.ts .../claude-code/runtime.test.ts  # 30 passed

Follow-up (not in this PR)

The deeper robustness gap — the provider boundary should degrade a
no-longer-readable image reference to a placeholder instead of sending a
broken block — is worth a separate change so a vanished /tmp file can't
permanently wedge a session.