effect-uai model-retry

Retry rate-limited / transient failures with exponential backoff. Non-retryable failures (ContentFiltered, AuthFailed, InvalidRequest, ContextLengthExceeded, ...) cross the boundary unchanged.

Reach for this when the user says any of:

• "Retry on rate limits / 5xx / timeouts"
• "Add exponential backoff to model calls"
• "Don't retry on auth failures or content filter rejections"

The retryable set