fix(plugins): preserve document path encoding

Keep OS-native path data intact across IPC instead of replacing non-UTF-8 bytes.

Refs #901

crates/ocs_plugin_api/src/ipc/protocol.rs

@@ -136,7 +136,7 @@ pub enum PluginRequest {
     CloseDocumentViewV4 { tab_id: u64 },
     /// V4: ask the host for the stable tab identifier of the active tab.
     GetTabId,
-    /// V4: ask the host for the filesystem path of the document in `tab_id`.
+    /// V5: ask the host for the filesystem path of the document in `tab_id`.
     DocumentPath { tab_id: u64 },
 }
 
@@ -162,8 +162,8 @@ pub enum PluginResponse {
     },
     /// V4: stable tab identifier of the active tab.
     TabId(u64),
-    /// V4: filesystem path of the document in the requested tab, if any.
-    DocumentPath(Option<String>),
+    /// V5: filesystem path of the document in the requested tab, if any.
+    DocumentPath(Option<std::ffi::OsString>),
 }
 
 /// Messages sent from the host to the plugin runner.