sloproxy

feat(executor): handle usage limit errors and enhance retry logic master

- Added `isCodexUsageLimitError` to detect and handle `usage_limit_reached` errors from Codex responses. - Updated `newCodexStatusErr` to treat usage limit errors as HTTP 429 with proper `RetryAfter` handling. - Enhanced test coverage to validate usage limit error handling, including reset time parsing and retry behavior. Closes: #2886