[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`

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -524,6 +524,11 @@ client_request_definitions! {
         response: v2::GetAccountRateLimitsResponse,
     },
 
+    SendAddCreditsNudgeEmail => "account/sendAddCreditsNudgeEmail" {
+        params: v2::SendAddCreditsNudgeEmailParams,
+        response: v2::SendAddCreditsNudgeEmailResponse,
+    },
+
     FeedbackUpload => "feedback/upload" {
         params: v2::FeedbackUploadParams,
         response: v2::FeedbackUploadResponse,