Declare the experimental plugin search API (#36402)

## What changed

- Add the experimental `plugin/search` request with search term, scope, working-directory, cursor, and limit parameters.
- Define paginated results that include each plugin's marketplace name and optional local marketplace path.
- Route the request through app server while returning method-not-found until the search implementation is available.

## Testing

- Add an app-server test that verifies `plugin/search` returns JSON-RPC error `-32601` with the expected not-implemented message.

GitOrigin-RevId: 755400468331f0887f7da7d933195c347aadf6d0

codex-rs/app-server-protocol/src/protocol/v2/mod.rs

@@ -18,6 +18,7 @@ mod model;
 mod notification;
 mod permissions;
 mod plugin;
+mod plugin_search;
 mod process;
 mod realtime;
 mod remote_control;
@@ -45,6 +46,7 @@ pub use model::*;
 pub use notification::*;
 pub use permissions::*;
 pub use plugin::*;
+pub use plugin_search::*;
 pub use process::*;
 pub use realtime::*;
 pub use remote_control::*;