[codex] Add owner nudge app-server API (#18220)
## Summary
Second PR in the split from #17956. Stacked on #18227.
- adds app-server v2 protocol/schema support for
`account/sendAddCreditsNudgeEmail`
- adds the backend-client `send_add_credits_nudge_email` request and
request body mapping
- handles the app-server request with auth checks, backend call, and
cooldown mapping
- adds the disabled `workspace_owner_usage_nudge` feature flag and
focused app-server/backend tests
## Validation
- `cargo test -p codex-backend-client`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server rate_limits`
- `cargo test -p codex-tui workspace_`
- `cargo test -p codex-tui status_`
- `just fmt`
- `just fix -p codex-backend-client`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fix -p codex-tui`
richardopenai