[codex] Expose unavailable app templates in plugin detail (#26317)

## Summary
- Adds `unavailable_app_templates` to the app-server protocol and
generated schemas/types.
- Parses plugin-service `release.unavailable_app_templates` in the
remote plugin client.
- Maps remote unavailable templates into app-server `PluginDetail`.
- Defaults local plugins to an empty unavailable app template list.

## Validation
- `just write-app-server-schema`
- `cargo +1.95.0 fmt --manifest-path codex-rs/Cargo.toml --all --check`
- `cargo +1.95.0 test --manifest-path codex-rs/Cargo.toml -p
codex-app-server-protocol schema_fixtures`
- `cargo +1.95.0 check --manifest-path codex-rs/Cargo.toml -p
codex-app-server-protocol -p codex-core-plugins -p codex-app-server`
- `git diff --check`

Note: default `cargo check` uses rustc 1.89 locally and failed because
dependencies require newer Rust, so validation was rerun with installed
Rust 1.95.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -643,6 +643,69 @@
       ],
       "type": "object"
     },
+    "AppTemplateSummary": {
+      "properties": {
+        "canonicalConnectorId": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "description": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "logoUrl": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "logoUrlDark": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "materializedAppIds": {
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        },
+        "name": {
+          "type": "string"
+        },
+        "reason": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/AppTemplateUnavailableReason"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "templateId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "materializedAppIds",
+        "name",
+        "templateId"
+      ],
+      "type": "object"
+    },
+    "AppTemplateUnavailableReason": {
+      "enum": [
+        "NOT_CONFIGURED_FOR_WORKSPACE",
+        "NO_ACTIVE_WORKSPACE"
+      ],
+      "type": "string"
+    },
     "AppToolApproval": {
       "enum": [
         "auto",
@@ -8690,6 +8753,12 @@
     },
     "PluginDetail": {
       "properties": {
+        "appTemplates": {
+          "items": {
+            "$ref": "#/definitions/AppTemplateSummary"
+          },
+          "type": "array"
+        },
         "apps": {
           "items": {
             "$ref": "#/definitions/AppSummary"
@@ -8738,6 +8807,7 @@
         }
       },
       "required": [
+        "appTemplates",
         "apps",
         "hooks",
         "marketplaceName",