[codex] Add installed-plugin mention API (#22448)

## Summary
- add app-server `plugin/installed` for mention-oriented plugin loading
- return installed plugins plus explicitly requested install-suggestion
rows
- keep remote handling on installed-state data instead of the broad
catalog listing path

## Why
The `@` mention surface only needs plugins that are usable now, plus a
small product-approved set of install suggestions. It does not need the
full catalog-shaped `plugin/list` payload that the Plugins page uses.

## Validation
- `just write-app-server-schema`
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-core-plugins`
- `cargo test -p codex-app-server --test all plugin_installed_`

## Notes
- The package-wide `cargo test -p codex-app-server` run still hits an
existing unrelated stack overflow in
`in_process::tests::in_process_start_clamps_zero_channel_capacity`.
- Companion webview PR: https://github.com/openai/openai/pull/915672

codex-rs/app-server/src/message_processor.rs

@@ -1102,6 +1102,9 @@ impl MessageProcessor {
             ClientRequest::PluginList { params, .. } => {
                 self.plugin_processor.plugin_list(params).await
             }
+            ClientRequest::PluginInstalled { params, .. } => {
+                self.plugin_processor.plugin_installed(params).await
+            }
             ClientRequest::PluginRead { params, .. } => {
                 self.plugin_processor.plugin_read(params).await
             }