Forward install attempt IDs for remote plugins (#37773)

## Why

Clients need to correlate a remote plugin installation request with a specific
installation attempt.

## What changed

- Add the optional `installAttemptId` field to `PluginInstallParams`.
- Forward the value as `install_attempt_id` in the remote plugin install POST
  body.
- Preserve the existing empty request body when the field is omitted.

## Testing

- Cover protocol serialization and remote install request bodies with and
  without an attempt ID.

GitOrigin-RevId: ffdceff4f9329b650848e6428a9ba39646c242c3

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

@@ -893,6 +893,9 @@ pub struct PluginInstallParams {
     pub marketplace_path: Option<AbsolutePathBuf>,
     #[ts(optional = nullable)]
     pub remote_marketplace_name: Option<String>,
+    /// Client-generated identifier used to correlate one installation attempt.
+    #[ts(optional = nullable)]
+    pub install_attempt_id: Option<String>,
     pub plugin_name: String,
 }