## Why
`slow_down` errors were treated as terminal server overloads, while exhausted credit balances and spend limits fell through to retryable stream errors.
## What changed
- Classify `slow_down` as a retryable rate limit in HTTP 503 responses and SSE failures, preserving the server message and parsing retry delays from SSE error messages.
- Map `credit_balance_exhausted`, `organization_spend_limit_exceeded`, and `project_spend_limit_exceeded` SSE errors to quota exhaustion so they terminate without retries.
## Testing
Extend tests to cover HTTP error classification and retryability, `slow_down` stream retry exhaustion and message-provided delays, and a single `UsageLimitExceeded` error with no retries for each quota error code.
GitOrigin-RevId: 7985eba71ba4a6fe897a45957e448fadd42f3963
31ffe2bc9a