[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/src/protocol/v2/plugin.rs

@@ -661,6 +661,7 @@ pub struct AppTemplateSummary {
     pub template_id: String,
     pub name: String,
     pub description: Option<String>,
+    pub category: Option<String>,
     pub canonical_connector_id: Option<String>,
     pub logo_url: Option<String>,
     pub logo_url_dark: Option<String>,