[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/schema/json/v2/SendAddCreditsNudgeEmailParams.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "AddCreditsNudgeCreditType": {
+      "enum": [
+        "credits",
+        "usage_limit"
+      ],
+      "type": "string"
+    }
+  },
+  "properties": {
+    "creditType": {
+      "$ref": "#/definitions/AddCreditsNudgeCreditType"
+    }
+  },
+  "required": [
+    "creditType"
+  ],
+  "title": "SendAddCreditsNudgeEmailParams",
+  "type": "object"
+}