[breaking] xAI silently redirects retired Grok model slugs to grok-4.3 at grok-4.3 pricing

From May 15, 2026, requests to the retired Grok slugs do not fail — they automatically redirect to grok-4.3 (grok-code-fast-1 redirects to grok-build-0.1) and are billed at the new model's rates ($1.25/$2.50 per 1M tokens) instead of the original model's. Code keeps working while the model and the price underneath it change — the exact stable-ID model swap the record exists to flag. Effective: 2026-05-15. Migration target: xai/grok-4-3. Affected: xai/grok-4-1-fast-reasoning, xai/grok-4-1-fast-non-reasoning, xai/grok-4-fast-reasoning, xai/grok-4-fast-non-reasoning, xai/grok-4-0709, xai/grok-code-fast-1, xai/grok-3. Severity: breaking. Verification: verified.