Surface admin-disabled remote plugin status (#20298)

## Summary

Remote plugin-service returns plugin availability separately from a
user's installed/enabled state. This adds `PluginAvailabilityStatus` to
the app-server protocol, propagates remote catalog `status` into
`PluginSummary`, and rejects install attempts for remote plugins marked
`DISABLED_BY_ADMIN` before downloading or caching the bundle.

This is the `openai/codex` half of the change. The companion
`openai/openai` webview PR is
https://github.com/openai/openai/pull/873269.

## Validation

- `cargo run -p codex-app-server-protocol --bin write_schema_fixtures`
- `cargo test -p codex-app-server --test all
plugin_list_marks_remote_plugin_disabled_by_admin`
- `cargo test -p codex-app-server --test all
plugin_list_includes_remote_marketplaces_when_remote_plugin_enabled`
- `cargo test -p codex-app-server --test all
plugin_install_rejects_remote_plugin_disabled_by_admin_before_download`
- `cargo test -p codex-app-server-protocol schema_fixtures`

codex-rs/app-server-protocol/schema/json/v2/PluginListResponse.json

@@ -38,6 +38,23 @@
       ],
       "type": "string"
     },
+    "PluginAvailability": {
+      "oneOf": [
+        {
+          "enum": [
+            "DISABLED_BY_ADMIN"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "Plugin-service currently sends `\"ENABLED\"` for available remote plugins. Codex app-server exposes `\"AVAILABLE\"` in its API; the alias keeps decoding compatible with that upstream response.",
+          "enum": [
+            "AVAILABLE"
+          ],
+          "type": "string"
+        }
+      ]
+    },
     "PluginInstallPolicy": {
       "enum": [
         "NOT_AVAILABLE",
@@ -299,6 +316,15 @@
         "authPolicy": {
           "$ref": "#/definitions/PluginAuthPolicy"
         },
+        "availability": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/PluginAvailability"
+            }
+          ],
+          "default": "AVAILABLE",
+          "description": "Availability state for installing and using the plugin."
+        },
         "enabled": {
           "type": "boolean"
         },