[codex] Propagate rate limit reached type (#18227)

## Summary

First PR in the split from #17956.

- adds the core/app-server `RateLimitReachedType` shape
- maps backend `rate_limit_reached_type` into Codex rate-limit snapshots
- carries the field through app-server notifications/responses and
generated schemas
- updates existing constructors/tests for the new optional field

## 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/GetAccountRateLimitsResponse.json

@@ -39,6 +39,16 @@
       ],
       "type": "string"
     },
+    "RateLimitReachedType": {
+      "enum": [
+        "rate_limit_reached",
+        "workspace_owner_credits_depleted",
+        "workspace_member_credits_depleted",
+        "workspace_owner_usage_limit_reached",
+        "workspace_member_usage_limit_reached"
+      ],
+      "type": "string"
+    },
     "RateLimitSnapshot": {
       "properties": {
         "credits": {
@@ -83,6 +93,16 @@
             }
           ]
         },
+        "rateLimitReachedType": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/RateLimitReachedType"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "secondary": {
           "anyOf": [
             {