[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/tui/src/chatwidget/tests/helpers.rs

@@ -1429,6 +1429,7 @@ pub(super) fn plugins_test_detail(
                 needs_auth: *needs_auth,
             })
             .collect(),
+        app_templates: Vec::new(),
         mcp_servers: mcp_servers.iter().map(|name| (*name).to_string()).collect(),
     }
 }