[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/ClientRequest.json

@@ -5,6 +5,13 @@
       "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
       "type": "string"
     },
+    "AddCreditsNudgeCreditType": {
+      "enum": [
+        "credits",
+        "usage_limit"
+      ],
+      "type": "string"
+    },
     "ApprovalsReviewer": {
       "description": "Configures who approval requests are routed to for review. Examples include sandbox escapes, blocked network access, MCP approval prompts, and ARC escalations. Defaults to `user`. `guardian_subagent` uses a carefully prompted subagent to gather relevant context and apply a risk-based decision framework before approving or denying the request.",
       "enum": [
@@ -2548,6 +2555,17 @@
         }
       ]
     },
+    "SendAddCreditsNudgeEmailParams": {
+      "properties": {
+        "creditType": {
+          "$ref": "#/definitions/AddCreditsNudgeCreditType"
+        }
+      },
+      "required": [
+        "creditType"
+      ],
+      "type": "object"
+    },
     "ServiceTier": {
       "enum": [
         "fast",
@@ -4982,6 +5000,30 @@
       "title": "Account/rateLimits/readRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "account/sendAddCreditsNudgeEmail"
+          ],
+          "title": "Account/sendAddCreditsNudgeEmailRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/SendAddCreditsNudgeEmailParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Account/sendAddCreditsNudgeEmailRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {