fix(plugins): gate on_load behind API v5

BREAKING CHANGE: plugins using on_load must target API v5 and declare api_version = 5.

Refs #901

crates/ocs_plugin_api/src/runner.rs

@@ -56,9 +56,9 @@ pub fn run(socket_name: &str, cdylib_path: &Path) -> Result<(), Box<dyn std::err
 
     if version >= 4 {
         let client = V4Client::connect_handshake(socket_name, &token)?;
-        // Give the plugin a chance to set up worker threads / cache the request
-        // sender before the host starts dispatching user commands.
-        invoke_on_load(&mut *plugin, &client, &interactive);
+        if version >= 5 {
+            invoke_on_load(&mut *plugin, &client, &interactive)?;
+        }
         run_v4(&mut *plugin, &interactive, client)
     } else {
         let client = IpcClient::connect(socket_name)?;
@@ -70,11 +70,17 @@ pub fn run(socket_name: &str, cdylib_path: &Path) -> Result<(), Box<dyn std::err
     }
 }
 
-fn invoke_on_load(plugin: &mut dyn BuiltinPlugin, client: &V4Client, interactive: &InteractiveRegistry) {
+fn invoke_on_load(
+    plugin: &mut dyn BuiltinPlugin,
+    client: &V4Client,
+    interactive: &InteractiveRegistry,
+) -> Result<(), Box<dyn std::error::Error>> {
     let mut proxy = client.plugin_host_api(0, interactive.clone());
-    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+    std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
         plugin.on_load(&mut proxy)
-    }));
+    }))
+    .map_err(|_| std::io::Error::other("plugin on_load() panicked"))?;
+    Ok(())
 }
 
 fn run_v3(