Function isRetryableUpstreamStatus

Decide whether an upstream HTTP status is worth retrying against a fallback model. Pure helper — callers compose their own retry strategy.

Retry on:

• 408 (request timeout)
• 429 (rate limited)
• 500/502/503/504 (provider unavailable / gateway / timeout)
• 529 (Anthropic overloaded)
• 404 (some providers transiently report a model as missing during incidents)

Do NOT retry on:

• 400 (bad request — fallback would fail the same way)
• 401/403 (auth — config issue)
• 422 (validation — body is malformed)