feat: Add plugin share checkout (#22435)

Adds plugin/share/checkout to turn a shared remote plugin into a local
working copy under ~/plugins/<name>.

Registers the copy in the managed personal marketplace and records the
remote-to-local mapping for later share/save flows.

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -1099,6 +1099,9 @@ impl MessageProcessor {
             ClientRequest::PluginShareList { params, .. } => {
                 self.plugin_processor.plugin_share_list(params).await
             }
+            ClientRequest::PluginShareCheckout { params, .. } => {
+                self.plugin_processor.plugin_share_checkout(params).await
+            }
             ClientRequest::PluginShareDelete { params, .. } => {
                 self.plugin_processor.plugin_share_delete(params).await
             }