[codex] Propagate plugin app categories (#27420)

## What
- Parse optional `.app.json` `category` overrides for plugin apps.
- Add nullable `category` to `AppSummary` and `AppTemplateSummary` in
the app-server protocol.
- Fall back from `branding.category` to the first non-empty
`app_metadata.categories` value when building app/template summaries.
- Regenerate schema/type fixtures and update plugin read/install tests.

## Why
The plugin details UI needs a normalized per-app category. Some apps
only provide their default category in metadata, while others need a
local `.app.json` override.

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

@@ -694,6 +694,12 @@
     "AppSummary": {
       "description": "EXPERIMENTAL - app metadata summary for plugin responses.",
       "properties": {
+        "category": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "type": [
             "string",
@@ -727,6 +733,12 @@
             "null"
           ]
         },
+        "category": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "type": [
             "string",