[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/tests/common/mcp_process.rs

@@ -58,6 +58,7 @@ use codex_app_server_protocol::PluginReadParams;
 use codex_app_server_protocol::PluginUninstallParams;
 use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::ReviewStartParams;
+use codex_app_server_protocol::SendAddCreditsNudgeEmailParams;
 use codex_app_server_protocol::ServerRequest;
 use codex_app_server_protocol::SkillsListParams;
 use codex_app_server_protocol::ThreadArchiveParams;
@@ -305,6 +306,16 @@ impl McpProcess {
             .await
     }
 
+    /// Send an `account/sendAddCreditsNudgeEmail` JSON-RPC request.
+    pub async fn send_add_credits_nudge_email_request(
+        &mut self,
+        params: SendAddCreditsNudgeEmailParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("account/sendAddCreditsNudgeEmail", params)
+            .await
+    }
+
     /// Send an `account/read` JSON-RPC request.
     pub async fn send_get_account_request(
         &mut self,